It's not surprising to see players check-call flop with top pair to get the c-bet then donk the turn to avoiding giving a free look at the river to a player who has an overcard, especially if the turn gave a potential c-betting hand a possible straight or flush draw. These players are usually unwilling to check-raise the flop with a one-pair hand, especially if they have a bad kicker.
He can definitely have a king there, dude. Because he checks the turn to you doesn't tell me a king is out of his range with the information you've given me.
The $5 open makes it easier to take some strong kings out of his range, guys. Still possible but less likely.
The 3b takes some crappy ones that he is messing around with out of his range, eff.
But now we know we can't as easily take the weak kings out with the spoiler. If he is calling with a5 he would maybe sometimes with a hand like the k9s, k10s etc.
