In my modest micro-staker opinion (giving opinions it is always a nice way to learn), I think he shoved the river because your range was pretty obvious to him on the river. Check turn, check river isnt strong enough to call a river shove and he realized this.

He puts you on either a missed FD or an overpair (UTG raise) and he can make most of this range fold to a river shove with anything. He noticed that you were willing to go to a cheap showdown or catch a draw on the river.

The problem with his line is that he is also playing with his cards faces up. With a a hand with showdown value, he would not shove river as he did (from which hands he is extracting value with such a bet if he has a reasonable hand here?).

Probably some players at these stakes have adjusted to this and occasionally are able to make shoves like this occasionally with strong hands. However, I think this is not be the case here.

Good call