Cave dwellers spawning inside the fort

[Laketa]

A couple of times already ive had cave dwellers such as rodent men spawn right on top of my farms and pens. They would kill everything before my military even gets a chance to go down to them. There is a clear entrance to the caves through which they can get in, but its not like they spawn at the edge of the map and move forward, they just spawn in the fortress. Is there something im doing wrong or is it just a stupid game mechanic?

[Fel]

They do spawn at the edges, even if it can sometimes look like they don't.
It is often pretty easy to not notice them moving in if there is a path, as you would only get notified when they are noticed by a dwarf (cancelling their current task) or when they start to attack something, which usually happens inside your fort unless you protected all possible entrances (or fully closed them all).

Rodent men are not sneaky, nor do they avoid traps, so military dwarves guarding the entrance(s) and/or traps should show you that they come in through there, assuming that there is no other path that you didn't notice of course.