Beware of the passive raiser

Recently, I played the following 5/10 Limit Holdem hand:

I open raise with 99 UTG+2 and Mr. Weak/Tight directly to my left thinks awhile and cold-calls; the SB and BB call.

FLOP: 7h-5s-4d, I lead with a bet, Mr. WT insta calls, SB calls, BB folds. So far so good.

TURN: 5d, SB checks, I bet and Mr. WT insta raises, SB folds and the action is back to me - time to go into the tank. I rule out an overpair b/c 99% of LHE players would have raised the flop to protect their hand. Checking his aggression stats, they are pretty low (around 1.5) or passive on all streets. For this reason, I also rule out a flush draw such as Ad-Qd. Passive players aren't capable of semi-bluffing a draw like that. I figure that he must have a made hand such as A5s (although calling 2 bets preflop there would be horrible) or more likely 77. I let the timer run down and eventually fold even though I'm getting 8.5:1 to call. He shows...........77, the Scottro.

Some of my friends disagree with my decision to fold, but I stand by it in this situation. When a straight-forward, passive player becomes aggressive, get ready to duck.