Why did I donk bet this?

[Spook]

loose passive preflop, bit more action postflop live 8/16 game.
I limp in early position with Q[img]/images/graemlins/spade.gif[/img]T[img]/images/graemlins/spade.gif[/img], six or seven to the flop.
Flop 6SB: J [img]/images/graemlins/club.gif[/img] 8 [img]/images/graemlins/spade.gif[/img] 9 [img]/images/graemlins/spade.gif[/img]
BB checks, I check, bet to my immediate left, everyone calls except the blinds.
Turn, four players 5BB: 3 [img]/images/graemlins/spade.gif[/img]
Hero checks, flop better checks, loose aggresive player bets, button folds, Hero goes for the overcall, flop better calls.
River 8BB: J [img]/images/graemlins/spade.gif[/img] for my gin card.
I donk bet.