Should have I dropped

[SirFoldsAlot]

This is the hand that ended my tourney play last night, so now I sit here wondering how I should have played this?


200/400 TourneyTexasHTGameTable (NL) (Tournament 802580) -
I have 6 [img]/images/graemlins/diamond.gif[/img] 6 [img]/images/graemlins/heart.gif[/img] in MP

UTG+1(T6105) calls 200, folded to me(T2925), I raise to 600, BB(T2230) cold calls and UTG+1 calls
** Dealing Flop ** : [ 7 [img]/images/graemlins/heart.gif[/img], 3 [img]/images/graemlins/heart.gif[/img], Q [img]/images/graemlins/spade.gif[/img] ]
checked to me and I bet 475, BB folds, UTG+1 calls

** Dealing Turn ** : [ 4 [img]/images/graemlins/spade.gif[/img]]

checked to me, I bet 600, UTG+1 calls

I would have dropped if he raised, and I planned to drop on the river if I saw any action

** Dealing River ** : [ 5 [img]/images/graemlins/spade.gif[/img] ]
checked to me, I go all in with my now straight, UTG+1 calls

UTG+1 shows A [img]/images/graemlins/spade.gif[/img] 3 [img]/images/graemlins/spade.gif[/img] for the runner, runner nut flush.

I figured he had middle pair and I could just outplay him, although I may have checked behind him on the river if I didn't make the straight. How does this guy call my flop bet with bottom pair?