Seem like a very standard cooler spot to me. The turn card kind of roped SB in, since he now beat the other two pair combos. He only lose to TT, 77 and QJ, and QJ should really only be QJ of hearts after calling the flop bet. Its also BTN vs. SB, so ranges should be wide, and I think, its completely fine for SB to try to go for the whole stack with AK. Hands like AQ, AJ, AT, A7s can still make a hero call on the river. Just unfortunate for SB, that he ran into it. And well played by BTN to just call down in position with bottom set.