So seeing I'm on the BB and I check turn, what do you think my opponent did wrong on the river?

PokerStars No-Limit Hold'em, $1.00 BB (6 handed) - Poker-Stars Converter Tool from FlopTurnRiver.com

SB ($100)
Hero (BB) ($100)
UTG ($120.75)
MP ($104)
CO ($198.80)
Button ($100)

Preflop: Hero is BB with 6, 3
UTG calls $1, 3 folds, SB calls $0.50, Hero checks

Flop: ($3) 2, A, 4 (3 players)
SB checks, Hero bets $2, UTG calls $2, SB calls $2

Turn: ($9) 9 (3 players)
SB checks, Hero checks, UTG checks

River: ($9) 5 (3 players)
SB checks, Hero bets $8, 1 fold, SB raises to $21, Hero raises to $97 (All-In), SB calls $76 (All-In)

Total pot: $203 | Rake: $3

Results:
SB had 3, 2 (straight, five high).
Hero had 6, 3 (straight, six high).
Outcome: Hero won $200