Once when I was feeding minions at the galactic wastes, a bubble popped and the minion came out. Is it totally random when this happens?

Yup, it's random (and rare).

Lol I didn't even know it was possible; I feed them all the time and it's never happened. So both minions I got were worth about 2k 😑

Yeah, I've only ever popped a few bubbles and have never got a minion worth anything. x)