50 NL have a set but CO takes a strange line

[forestwithin]

CO is 62.5/25/2.2 but I only have about 20 hands with him.
MP2 is 43/5/1.8 after a similar amount of hands.
I'm calling on the button here looking to take the pot heads-up depending on the flop, or in the hopes of hitting a set, but several other people come along.


PokerStars No-Limit Hold'em, $0.50 BB (9 handed) internettexasholdem.com

SB ($39.90)
BB ($88.40)
UTG ($15.40)
UTG+1 ($5.10)
MP1 ($10.35)
MP2 ($48)
MP3 ($50.25)
CO ($53.60)
Hero ($49.25)

Preflop: Hero is Button with 4[img]/images/graemlins/heart.gif[/img], 4[img]/images/graemlins/club.gif[/img].
<font color="#666666">2 folds</font>, MP1 calls $0.50, MP2 calls $0.50, <font color="#666666">1 fold</font>, <font color="#CC3333">CO raises to $2.5</font>, Hero calls $2.50, <font color="#666666">2 folds</font>, MP1 calls $2, MP2 calls $2.

Flop: ($10.75) 8[img]/images/graemlins/diamond.gif[/img], 5[img]/images/graemlins/club.gif[/img], T[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(4 players)</font>
MP1 checks, MP2 checks, CO checks, Hero checks.

Turn: ($10.75) 4[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(4 players)</font>
MP1 checks, <font color="#CC3333">MP2 bets $6</font>, CO calls $6, <font color="#CC3333">Hero raises to $18</font>, MP1 folds, MP2 calls $12, <font color="#CC3333">CO raises to $51.1</font>, Hero folds, MP2 calls $27.50 (All-In).

River: ($125.35) K[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(2 players, 1 all-in)</font>

Final Pot: $125.35

After he checks the flop, then calls a bet on the turn, then raises my raise, it feels like a monster. I'd have to imagine that jacks-aces would bet the flop and since there's no flush draw, a set might play like this? People like to slowplay at this level. I timed out while thinking that I was probably beaten, and I never look to fold sets but it just seems like this may be a place to do so, but I was wondering what you all thought.