The Sleepers’ motivations have always been a little obscure, I think. Mostly the ships we see behave in pretty simple ways, and kind of seem to be mostly a maintenance system with guns.
… not to say they don’t use them for military stuff, of course.
Anyway-- it’s possible that the Sleeper craft are busy with other stuff, but scramble in response to an alarm or warning of some kind. It’s also possible that they’re intentionally waiting in ambush.
If you figure out a way to ask, I’m pretty sure a lot of people would love to have a chance to talk with them. It’s not like they’ve been explaining anything they do.