I just don't understand...

[FlopComesBricks]

Pretty common at the Party $25/50 NL. Which that was, yes? Alot of tables are so weak-tight and get scared poopless from any kind of raise that its folded to the blinds. So UTG wants action on his mighty aces and keeps praying you'll bet so he can raise you.

On the river he's so weak-tight he's worried you caught that 3 or worse the baby-straight so he checks behind.

Or it was an image check. He wants you to think he's retarded.