KK on dry flop

[True]

500 nl, eff stacks 500, vill unknown

I raise 20 withKK UTG, vill ccs UTG+1, 2 other callers

flop - 963 rainbow
check to me, i bet 59 into 79, he raises to 179, others fold.

My thoughts here are, flop is dry so why raise a set especially with 2 guys behind