Happened in my previous playthroughs, happened in the current one.

Insects just show up without warning, I only find out because COLONIST NEEDS RESCUE, I look over and it's a huge insect pile dismembering the colony founder who went out for a walk.

Right now I discovered the insect spawn only because battle music started playing on save load.

Also insects spawned outside the base even though nothing in my base is under the overhead mountain. I made very sure careful about that.