You can usually blame Old French and / or Latin. I think in this case it’s “Caboche” / “Caboce” from the French, which both mean “head”. Plus there’s “Caput” in Latin which also means “head”. A head of cabbage! Knda makes sense now.
Discussion
I love how humans went crazy with the mustard plant. It's like half of our veggies.
Yeah mustard plants are a species within the Brassica / cruciferous genus, which includes other species like cabbage, cauliflower, broccoli, brussel sprouts etc. Some evidence of anti-cancer properties too.