NL noob hand: River checkraised out of the blue

[Magior]

50NL, effective stacks about $65

Villain is 35/5 over 70 hands who loves to open-limp and slowplay

Folded to SB who limps. I raise to $2.50 with 9 [img]/images/graemlins/diamond.gif[/img] 5 [img]/images/graemlins/club.gif[/img] in the BB

Is my raise bad? Standard?

Flop (Pot $5) 2 [img]/images/graemlins/heart.gif[/img] K [img]/images/graemlins/spade.gif[/img] Q [img]/images/graemlins/club.gif[/img]
SB checks, Hero bets $4, SB calls

Turn (Pot $13) 5 [img]/images/graemlins/spade.gif[/img]
SB checks, Hero checks

River (Pot $13) 9 [img]/images/graemlins/heart.gif[/img]
SB checks, Hero bets $7, SB calls $7 and raises $18

Okay, WTF?!!?

Why would he do that? If he has a monster he was slowplaying (or JT), wouldn't he want to lead the river after my turn check? There's no indication of me wanting to bet the river. Are plays like this common in NL and if they are, are the more likely to be slowplayed monsters or desperate bluffs.

Finally, should I call? I've never seen this guy bluff but then again, I haven't seen this guy play that much.