set, but werid situation

[udrazer]

the villian is pretty standard, and i was playing multi tables, so i dont have too much of a read on villian, but i dont think villian is an aggresive player at all. i was in small blind, so i didnt raise with my small pocket pair.
FullTiltPoker Game #1343605624: Table Key Vista (6 max) - $0.25/$0.50 - No Limit Hold'em - 22:34:35 ET - 2006/12/02
Seat 2: villian ($87.50) (villian is on button)
Seat 3: player x ($45.25)
Seat 4: hero ($51.80)
Seat 6: BB ($100.35)
hero posts the small blind of $0.25
BB posts the big blind of $0.50
The button is in seat #3
*** HOLE CARDS ***
Dealt to hero [4s 4h]

villian calls $0.50
player x folds
Link9 calls $0.25
BB checks
*** FLOP *** [6d 4c 8h]
hero bets $1
bb folds
villian calls $1
*** TURN *** [6d 4c 8h] [Kc]
hero bets $2.25
villian calls $2.25
*** RIVER *** [6d 4c 8h Kc] [Jc]
hero bets $7
villian raises to $83.75, and is all in
hero?

[DXX]

he probably backdoored the flush with Ac8/6c . I'd need proof that he's a total moron b4 calling this

[lacrymosa]

fold. looks like a backdoor flush, or even a higher set. Either way I dont' see KJ doing this.

[Big Poppa Smurf]

yeah honestly i don't fold sets and overpairs much, but if i'm villain the only times i would do this exact move is with a backdoored flush thinking you couldn't figure out what happened

[udrazer]

ya, i actually folded, i didnt even notice a backdoor flush as i had other thinking situations at that moments at other table. i thought he slow played a flopped straight, i really thought he had a big made hand, figured he wouldnt bluff at that spot, and 2 pair was very unlikey.

[udrazer]

and thank you all for analyzing, i think villian caught flush on river is the most reasonable answer.