[Rotating Rabbit]

I call here. It appears to villian that you are on a re-steal with AK, that you didnt want to do a continuation bet into 4 players but when you saw his weak bet you figured he would fold if you raised. Turn comes low, since you have only 40 bb left and there's 40 in the pot the only sensible bet is all-in which he makes. I think you're ahead, because he doesnt expect you to call.

Edit: What turnip says below is another reason he doesnt expect you to call, if you did indeed have an overpair you would probably bet the turn.