Why does he raise my KK???

[Beck]

first hand, blinds 10/20, 1500 stacks.

Hero is dealt KhKc UTG1, raise to 100. MP1, MP2 CO and BUT call.
POt is 530 and flop is Jh6h6c. I bet 400, MP1 folds, Mp2 calls, CO goes all-in, BUT folds, hero ???

I called, expecting to see AJ.