turn brings TPTK - but crazy action in multiway pot (NL400)

[Fingerswinger]

i wanted to raise preflop, but another table was taking my attention and i clicked call before i realized the situation.

Party Poker
No Limit Holdem Ring game
Blinds: $2/$4
5 players
Converter

Stack sizes:
UTG: $168
CO: $595.20 <font color="#0000cc">30/12/4.0 (seems a bit maniacal)</font>
Button: $213.30 <font color="#0000cc">19/12/2.4 (reasonable player i think)</font>
SB: $81.20
Hero: $463 <font color="#0000cc">(TAG, haven't done anything out of line)</font>

Pre-flop: (5 players) Hero is BB with A[img]/images/graemlins/club.gif[/img] Q[img]/images/graemlins/club.gif[/img]
<font color="#cc0000">UTG raises to $8</font>, CO calls, Button calls, SB folds, Hero calls.

Flop: 4[img]/images/graemlins/heart.gif[/img] 6[img]/images/graemlins/diamond.gif[/img] T[img]/images/graemlins/club.gif[/img] ($34, 4 players)
Hero checks, UTG checks, CO checks, Button checks.

Turn: Q[img]/images/graemlins/diamond.gif[/img] ($34, 4 players)
<font color="#cc0000">Hero bets $20</font>, UTG calls, <font color="#cc0000">CO raises to $96</font>, Button calls,

Hero?


COs massive raise doesn't really worry me that much, but buttons call does.
but then again what could he have that beats me after he checked the flop? slowplayed set?