The cabbage is 'bolting', which means it's sending up a column on which to produce flowers and then seeds. There is nothing you could have done to prevent it. Just enjoy the show!

Rainbow Gardens Nursery on Thousand Oaks here in San Antonio has a bunch of bolting cabbages lined up out in front. They were stunningly beautiful with their tall yellow flowers! I wish I had taken a picture!

Thanks Ragna for writing "Just enjoy the show" rather than "remove them immediately".

I was wondering too, as this was my first year trying them for winter color. Neil Sperry's column reported how they're doing after our early March freeze. Assessing the Impact His succinct wording (varies slightly from year to year) is humorous quite often.

I let some grow out, to see what the flowers and seedpods look like. As Brassicas go, these little ones are nice. Bees and butterflies are visiting. After the wind partly toppled one (planted in loosely broken up clay) it looks even more Dr Seuss-ish as it curves back upward. Initially during the drab overcast days when ambient daylight brightness is so much lower, the dark yellow flowers looked almost fluorescent.

My two plants are bolting now, too: https://imgur.com/rqJBT5G