They may be preparing a mega attack to overwhelm those turrets. Other than that, I don't know. They do a similar thing on my game too, but they group once then attack. Try pissing them off with some low-level turrets so that they attack.

I'd really like to see some "nests" underneath biter spawners, with some new, powerful enemies guarding any eggs, and you may need to destroy the nest to wipe out the base, or it will regenerate. and maybe some artefacts for killing the guardians.
Another idea, maybe change the tunnel bore to a ...