I am splatting south as I write this and here are a few observations...
They appear randomly, there is no time pattern. Sometimes it is just one or two minutes, sometimes 10 or 15 (or they appear on other places, can't be everywhere simultaneously). It seems, however, that they come in small squads of under 10 individuals.
It appears that they can penetrate diagonally from the front line. 10522 was infested, whereas 10487 and 10521 were not at one time. Or maybe it was not the case; it is possible that somebody else killed some before I came.
What also happened to me: I cleared 10344, disengaged from the last fight and then got distracted for a few minutes. When I came back at the screen, I clicked to move north and found myself immediately in another fight without warning. The very same square was invaded again in the meantime! It goes without saying that I was low on hp's from the previous fight and died immediately. So be careful when patrolling, regenerate and/or refresh screen before you move.
Our ungrateful monsters which were given free range on ta lie low on infested squares
and
on the squares that are adjacent to them. When the surrounding is cleared, they wake up and attack us, apart from disturbing reports of extinction in some areas. Monstalogists should perhaps investigate.
EG