QQ, strange action

[jakbse]

grunch
I'd guess SB was semi-bluffing with a flush draw, probably one with outs to overpair worth to protect. When he missed he could beat only a bluff and decided to fold the river. I think UTG had either a lower PP than yours or AK. The reason for saying is that I would expect him to cap the flop or raise the turn with a higher PP.
Given that I think you should have raised the turn.