Cabbage Problem

What am I doing wrong? Most of my cabbage heads split before I pick them, why is that and what can I do?

Don't ask me why this works, my mother taught me this. If your cabbage head starts to split then take the head and give it a turn of about 1/2. If I remember correctly it is to break off some of the roots (but not all). I just can't remember why we do this.