No comprendo

I don’t understand anything about this hand. I had a sense about the player and he seemed weak so I took him all the way to the river. And I was right about him being weak, mostly. Someone explain how he stayed in with me.  I know I should have folded to the river raise, but I still really felt like he had nothing and had just decided to push me.

Poker Stars, $0.10/$0.25 NL Hold’em Cash Game, 7 Players
LeggoPoker.com – Hand History Converter

BB: $28.65

UTG: $24.55

UTG+1: $19.95

MP: $23.30

CO: $27.70

BTN: $11.75

Hero (SB): $15.75

Pre-Flop: 9 9 dealt to Hero (SB)

5 folds, Hero raises to $0.75, BB calls $0.50

Flop: ($1.50) 8 Q 5 (2 Players)
Hero bets $1, BB calls $1

Turn: ($3.50) 4 (2 Players)
Hero bets $2, BB calls $2

River: ($7.50) 8 (2 Players)
Hero bets $2.50, BB raises to $8, Hero calls $5.50

Results: $23.50 Pot ($1.15 Rake)
BB showed 8 9 (three of a kind, Eights) and WON $22.35 (+$10.60 NET)
Hero mucked 9 9 and LOST (-$11.75 NET)