BAD BEAT

I hold A9.

The flop is 239, all diamonds. My ace is a diamond.

It checked around and the button bet $2. I called as did others.

The turn is a black 7.

It checked around and the button bet $4. I raised to $20. He calls.

The river is the J of clubs. I check. He checks.

He shows QJ black and takes the pot.

Question for Tom: why did he put in that $16 on the turn?