From the Desk of the Poker Philosopher

Curious: why don't you expect a random player with Ax to bluff in that situation?
 

Not sure what you mean. I do think Ax in his range. There are two Aces and one K in the deck, which makes it close, if I assume that he always plays Ax this way. Mix in the possibility of AA and the fact that I think Kx is more likely than any given combo of Ax, and I found a fold.
 

Oh, I understand that. I was specifically referring to this part of your original post:

"I knew nothing about this player, but I don't really ever expect a random to be bluffing here."

I guess this means that you were weighing between a semi-bluff (Ax) and quads, but you had ruled out a total bluff (say, QQ)? And if so, why?
 

Most people, even a lot of regulars, are simply too timid to make a huge double barrel bluff with air. It's also significant here that this turn bet was much bigger than it needed to be. A player who was going to bluff here would probably bet 60-80% of the pot at most. Hitting the pot button again is something overexcited donks do. Given what I said before about people being afraid to bluff, I definitely don't think a random is going to be cavalier about betting more than necessary even if he were bluffing.