There are a lot of things that can happen in Dinosaur Simulator, but there are some things that happen a lot, uncommonly or by trolling/KOSing players. Note if you find these, don't try to stop them instead stay away.
Sometimes barosaurus ( Or other strong dinosaurs ) can group in one area, usually at mid, and kill everything that comes near them. These rampages are very hard to stop, but with enough strong dino like Megavore/Albino Terror. It might take time if there are a load of elder Baro's, The best thing to do is leave them alone or find a new server. Somewhat common event
A common occurrence is small dinosaurs, usually baby triceratops although it can be arizonas or achillos, will dart up to players, attack them then retreat, trying to kill the players. These can be stopped by picking them up and biting them, or using AOE ( Stomp ), by pressing "1" whenever they come close. Somewhat common event.
Usually in the winter, many players begin to starve or die because ferns are almost nowhere in the map to be found and tree leaves are inedible. and it gets worst when the water freezes and they are stuck under it. This is great for carnivores or omnivores, as their deaths provide plenty of easy food for them to eat. It is highly suggested that you switch to be a carnivore or omnivore dinosaur so that you may live or to fill up your hunger bar during Autumn. Often common in most servers
Carnivore War Zone
This usually happens when two strong dinos, like Indoms/albino terrors and megavores kill everything within sight. These usually occur near the middle of the map, and can happen with any strong dinos like the giga or Tyrannosaurus rex. Unlike the Barosaurus rampages, these are much more easily stopped, since any player with a strong dinosaur like a sauro, baro, or brevip, could put an end to this. Any other player with Megavore or Albino terrors could also stop this by killing them. However, this is much harder when there are clans or packs of carnivores such as the giga. Killing a whole pack would require much more players. Common event
This usually happens when there are less players on a server, and the only available food source are little green lizards and baby triceratops' and other small dinosaurs. An easy way to prevent this from happening is to simply join another server with more people. Somewhat common
These events usually occur when one pack of dominant dinosaurs, usually carnivores, along with a few baros, kills everyone they see. They constantly move around the map and kill everything that moves for fun. Few players try to stop them, but stopping a pack of eight or more gigas, albinos, or megavores is pretty hard. These packs lure players near them by kindly saying "hi" or "wanna join?" and then brutally killing them as soon as they come close. But, unlike the barosaurus rampages or Carnivore war zones, these Death packs, nearly never die down! Even if almost half of them are killed by a competing pack, most players have an extra elder or two, and would just work from there. The only way for them to stop is usually if too many pack member leave, or when the pack breaks or all another player have elder Barosaurus Somewhat common, particularly happens to weaker dinosaurs
Middle Lake Ambushes
Sometimes an powerful aquatic creature(usually a Mosasaurus or a Pliosaurus)station themselves at mid and ambushes dinosaurs that come close to drink.This is particularly lethal to small or baby dinosaurs,that are killed in seconds or grabbed in their Jaws and dragged into the depths.With the addition of the subterranean tunnel to mid lake these attacks are becoming more and more common.Mid ambushes can be easily stopped if you have a elder baro,but this should not be attended if there are several aquatic predators,let alone a whole pack.Sometimes the responsibles can also be Sharks, Crocodiles,Prognathodon,and rarely, Tusoteuthis. Pretty common
Hunting for sport
Many players will often hunt for sport, (sometimes) bragging about what they kill. Usually, they hunt weaker dinosaurs, leave them there (if they pick you up but don't eat you, they may be hunting for a pack or making a food storage, not hunting for fun. Also they may get attacked by a dino, so they leave you to defend themselves), and hunt more; with little to no reason except "for fun". Since they typically hunt alone, it's easy to kill one with a dinosaur at their strength level. (Don't mistake these for dinosaurs hunting for their pack-- often, packs have to constantly hunt to keep their members alive.) Most hunters get bored after a while and leave the server, or get attacked and die (or leave the server, too.) Somewhat common
Death from above
Sometimes Quetzalcoatlus (Particularly fantasy and kaiju) or Hatzegopteryx players get a little trolly and stalk packs of small or baby dinosaurs. They then swoop down and grab one of them, proceeding to fly away with them in the mouth and killing them in the air. They also attack bigger dinosaurs babies and will kill them if the parents don't suceed in fending them off. This can be a difficult task, as one hit from the flying predator is enough to grab hold of the young one, and getting out of reach is an easy task for them after that. It is lucky that Quetzalcoatlus and Hatzegopteryx usually are pretty skittish and don't fight big dinosaurs.Somewhat Common
Sometimes the Two dominating packs kill the leader of both packs, and because of this, They start a war to settle their differences. What will usually happen is everybody in both packs will get their strongest dinosaurs, and some members will get either a Range Glitch dinosaur or a skin with a buff, or get a Mosasaurus or Pliosaurus into the wall. They either fight in the Middle of the map, or in some other place, either the island with the tree on it, as to not get attacked by other dinosaurs that cannot swim well, or the tiny forest. If you get caught up in the middle of the battle zone, it is recommended to leave as soon as you can. If you're there before when you see them run in then run out of where they are or swim/fly out. Slightly Uncommon
This one is exactly what it says on the tin: Dinos who nibble you. Trikes are very common, as are ornis. More experienced nibblers will use arizona or achillo. Pteras, especially decomposers, are also common.