I just had an infestation spawn under open sky. There are no mountain or rock roofs, and as far as I remember, there never were in that area. There are some constructed roofs nearby, but not over the spawned insects. These bugs appeared between the structural beams of my spaceship.
My only mods are, as far as I know, not related to gameplay: Blueprints, More Harvest Designators, and P-Music.
[Save File (https://www75.zippyshare.com/v/q8SvhEwD/file.html)]
[Player.log (https://0bin.net/paste/b8Ji8plitLZ4duap#z1EoDG9eoxBnEGJK2xQUU3LC+is4KtB2mnLqfz08Dpk)]
[attachment deleted due to age]
They spawned because you are deep drilling there. You should have gotten a notification (that may be in the History/Messages tab) stating as such.
Deep drilling carries a chance of a small infestation.