NL2, 6-max, bad board vs tiny river bet

Pacific Poker - $0.02 NL (6 max) - Holdem - 3 players
Hand converted by PokerTracker 4

BB: 23 BB (VPIP: 62.50, PFR: 0.00, 3Bet Preflop: 0.00, Hands: 8)
Hero (BTN): 121.5 BB
SB: 101 BB (VPIP: 13.33, PFR: 6.67, 3Bet Preflop: 0.00, Hands: 15)

SB posts SB 0.5 BB, BB posts BB 1 BB

Pre Flop: (pot: 1.5 BB) Hero has A♠ T♠

Hero raises to 2.5 BB, fold, BB calls 1.5 BB

Flop: (5.5 BB, 2 players) T♦ 6♣ 8♦
BB checks, Hero bets 2.5 BB, BB calls 2.5 BB

Turn: (10.5 BB, 2 players) K♥
BB checks, Hero bets 5 BB, BB calls 5 BB

River: (20.5 BB, 2 players) Q♣
BB bets 5 BB, fold,

Preflop: obvious, I think
Flop: I bet for value, chargind flush draws, lower T and pocket pairs. Come to think of it, I think I could have bet more. Even though the villain is short stacked, I'd go all-in with him at this point.
Turn: at the moment the hand was played, my thought process was the same as the flop. Afterwards, I think maybe only Tx would have called and K♦ x♦
River: here, I am pretty sure I am beat. He didn't mind my two bets and he is willing to bet on a high card. What I am curious about is what he tries to achieve with his tiny bet.

Opinions?