Explain this hand

[noles321]

Playing 5/10 no limit in Biloxi. The average stack is 4k and it is an agressive gambling table. We were playing a mandatory straddle so it was really 5/10/20. Second position raises to 50 five people call and I am on the button with KK I reraise to 350.00 the orginal raiser calls. This player is an older guy but an agressive gambler. The flop comes 3 7 j rainbow and he moves 'all in' right into me. I can't figure out what is going on it felt like a move he planned before he even called. It is 3500.00 to call??????????? I have never been that confused at what was going on... Give me your thoughts. I felt I was ahead..