3/6 Weird Check-raise All in on River

[m_reed05]

Villain is 22/18/4.5
I only have 273 hands on the villain, and I'm not sure how many I had at the time this hand occurred. I do know that up to this point, he had never check-raised the river, in fact he had never check-raised any street up to this point. And when I bet the river, he insta-check-raised all in--there was no hesitation at all. His line just makes no sense to me.

I have two thoughts from my experiences: 1) There aren't that many people out there at these stakes tricky enough to check-raise bluff the river. 2) In my experience, people with real hands are more likely to take their time when betting and check-raising, and instant bets and check-raises are much more likely to be bluffs, especially on the river.

So?

Full Tilt Poker, $3/$6 NL Hold'em Cash Game, 5 Players
LegoPoker Hand History Converter

Hero (CO): $1,314.45
BTN: $120
SB: $1,053.65
BB: $609
UTG: $600

Pre-Flop: K[img]/images/graemlins/diamond.gif[/img] K[img]/images/graemlins/club.gif[/img] dealt to Hero (CO)
<font color="red">UTG raises to $21</font>, Hero calls $21, BTN folds, SB calls $18, BB folds

Flop: ($69) 5[img]/images/graemlins/diamond.gif[/img] 9[img]/images/graemlins/heart.gif[/img] 9[img]/images/graemlins/spade.gif[/img] (3 Players)
SB checks, <font color="red">UTG bets $50</font>, Hero calls $50, SB folds

Turn: ($169) 3[img]/images/graemlins/club.gif[/img] (2 Players)
UTG checks, Hero checks

River: ($169) 8[img]/images/graemlins/spade.gif[/img] (2 Players)
UTG checks, <font color="red">Hero bets $90</font>, <font color="red">UTG raises to $529 and is All-In</font>, Hero ??[/i]