Villain is 50/3/1.0 after 400 hands and has been a calling station with no noticable aggression. What could he have here?

Party Poker 10/20 Hold'em (6 max, 6 handed) pokerhand.org hand converter

Preflop: Hero is MP with A[img]/images/graemlins/diamond.gif[/img], K[img]/images/graemlins/club.gif[/img].
<font color="#666666">1 fold</font>, <font color="#CC3333">Hero raises</font>, <font color="#666666">1 fold</font>, Button calls, SB calls, <font color="#666666">1 fold</font>.

Flop: (7 SB) 5[img]/images/graemlins/club.gif[/img], 4[img]/images/graemlins/spade.gif[/img], A[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(3 players)</font>
SB checks, <font color="#CC3333">Hero bets</font>, <font color="#CC3333">Button raises</font>, SB folds, <font color="#CC3333">Hero 3-bets</font>, Button calls.

Turn: (6.50 BB) 5[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">Hero bets</font>, <font color="#CC3333">Button raises</font>, Hero calls.

River: (10.50 BB) 2[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
Hero checks, <font color="#CC3333">Button bets</font>, Hero calls.

Final Pot: 12.50 BB