It sounds like they're getting ready to flower & set seeds.

I once let some go to seed (mainly to see how weird they would look) & later I had a million tiny cabbages!

Mine did that too this year ... I have no idea why. It was an unusually hot & dry summer.

What sort did you get? Were they spring cabbages or a maincrop? And how long have they been in for?

With the weird weather we've been having all sorts of things are bolting for the sky. I'm expecting Jack and the giant to come down the silverbeet any day now!