Two Plus Two Older Archives - 2/4 NL. Someone explain this to me

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2/4 NL. Someone explain this to me

2/4 NL with a 300 Max buy in. Everyone relevent has around 300, except the original raiser, who has 25.

UTG minraises and 3 guys call. The sb calls and I call with 7s3s in the BB. I had been trying to move away from calling this crap in the BB, but for a minraise in a big pot, I thought it was ok.

Flop is 9s4c5s and the SB leads out for 15. He had been leading a lot with weaker hands, and I thouhgt I could get it heads up with the nearly all in guy behind me, so I raised to 75. UTG called for 17 and the SB called.

Turn was an offsuit 2. (The pot was about 100 and the side pot was 90 or so). The SB checked and I bet 200 of my remaining 225, he moved in and I called.

The river blanked and he won the big side pot with A9(UTG won the small main pot with TT).

After the hand I asked how he could such a call with A9 and he said he was trapping me with the hand. Someone else chimed in and said that against me it was a close decision, but wouldn't tell me why. I have no idea why that is, and feel like I am missing a big concept here. I feel like he donked off all his money with top pair on a low board, but he and others seem to think that against me it was a good play. Why is that, and how can it be applied to other players?

Note that I would play a set(or even an overpair) in the same way. I would raise the flop since I dont want to give a cheap draw, and if he has a made hand, I want to start building the pot. On the turn, I would bet all in, or nearly all in, as the board can become much scarier on the river with a lot of cards(if he has a made hand), and if he has a draw, I want him to fold).

Thanks,
Jason