A8o NL 100, flop 2 pair, wtf on the turn

[DaGrunt]

No reads on the guy, seems decent but a bit too passive. Now on to the hand. I flop two pair and value bet it happily until something funky happens on the turn. What the hell could he do this with? What's the best approach here?

Full Tilt No-Limit Hold'em, $1 BB (5 handed) Full-Tilt Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

Hero ($127.40)
SB ($97.50)
BB ($104.05)
UTG ($59)
MP ($105.90)

Preflop: Hero is Button with 8, A.
2 folds, Hero raises to $3.5, 1 fold, BB calls $2.50.

Flop: ($7.50) 8, A, Q 2 players)
BB checks, Hero bets $4, BB calls $4.

Turn: ($15.50) 6 (2 players)
BB checks, Hero bets $9</font>, BB raises to $25