Most plays like this one occur when the opponent has a mid pair but an overpair nonetheless. You need to extract as much value as soon as possible because the turn can be a high card than his pair (in this case, an ace). When the ace hits, he no longer has an overpair and will not be willing to put too much more in the pot.

I have many rules for playing poker. My #2 rule is: Don't slowplay against the weak.