Healthy river suckout against overbet

[Atropos]

Party Poker No-Limit Hold'em, $ BB (6 max, 6 handed) converter

SB ($102.3)
BB ($12.7)
UTG ($138.8)
MP ($203.06)
CO ($160.42)
Hero ($136.75)

Preflop: Hero is Button with 9[img]/images/graemlins/club.gif[/img], K[img]/images/graemlins/club.gif[/img]. SB posts a blind of $0.5.
<font color="#CC3333">UTG raises to $2</font>, <font color="#666666">2 folds</font>, Hero calls $2, <font color="#666666">1 fold</font>, BB calls $1.

Flop: ($6.50) 8[img]/images/graemlins/club.gif[/img], A[img]/images/graemlins/heart.gif[/img], J[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(3 players)</font>
BB checks, UTG checks, <font color="#CC3333">Hero bets $3</font>, BB folds, UTG calls $3.

Turn: ($12.50) 9[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
UTG checks, Hero checks.

River: ($12.50) 9[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">UTG bets $15</font>, Hero calls $15.

Final Pot: $42.50

He bets out of the blue. Normally means weakness, but what hand could he have he could call down a reraise without having me beat?