Personally, I believe there's a bug in the raid system. Be careful complaining about it, though, the community for this game takes poorly to any claims that there might be something broken with the difficulty.

I made a post a couple weeks ago about having something like 30 greydwarfs/shamans/brutes spawn in over the course of about 8 seconds (the length of time between seeing "The Forest Is Moving" and stepping out my front door, followed by getting stun-locked in my bed while they killed me repeatedly and destroyed my house), and got flamed so hard a mod locked the thread.

If you want to avoid waves of "git gud, scrub", just chalk it up to finding a bad spot in the code, and hope it doesn't happen again.