Nit pushes on turn - overpair

Originally Posted by Harry

Villain is 22/1.5/0.6

Flop: 2 5 6 ($2.75, 2 players)
Hero bets $2, UTG calls.

Turn: 6 ($6.75, 2 players)
Hero bets $5, UTG raises all-in $20.65, Hero calls.

???why???