There are 'events' that can occur randomly if you hang around too long, these events spawn in mobs to attack your base and the types of mobs is based on how far you progressed.
YT'rs mess about a lot because its fun to chill and just work on the base a bit, mix in some hunting, or resource collecting, or explore new biomes etc etc.
It isn't a action game, the combat is decent but not the focus.