Each time I play the Annulus map set at around the 30 minute mark, a massive pirate fleet of 50 plus ships attacks one of my planets. This is not balanced.
This has occurred at least eight times in a row.
I've encountered this a few times as well (and yes they were absolutely pirates as they have unique ships). It seems to happen mostly when you take a planet bordering the pirate base. They swarm in with a HUGE fleet, destroy your planet, then leave. Best way to discourage that is bring your fleet, park them there for a bit and build up some defensive turrets, healing bays, and then a star base. Once all that is complete they won't invade anymore (and if they do, they get chewed to bits).
Don't feel bad though, they also do the same thing to the AI (it's hilarious watching the AI try to take a planet bordering the pirate base and then they get mobbed lol).