I had a similar thing happen to me once. Me and two others were about evenly stacked, but I think I was a little ahead. I was bb with 83s, short stack was utg was all in with a 2x raise. Fold to me, and then I fold, table explodes with "waa waa, you should have called with anything, waa waa."
Seriously, sure the short stack's shoving with about anything... but most things are better than 83s, and I'd imagine he looked at the cards and said "probably better than what I'll get when I have to put in bb next round." So I'm thinking, he's got this chance and the bb to do anything, so 50/50 chance this is better... probably has a card in the top 50%... at least a hand that's better than 83s.
People get pissy when you don't do what they want you to do. Sorta like those guys who get all upset when you double down on 13 (don't do that, for your own sake). It's not collusion, it's not unethical, it's you making the decision that is in your best interest. The fact that you're doing something that is not in the table's interest is evident enough of not colluding. These are the same people who get pissed when you bet the turn after the short stack is all-in pre-flop. "Waaaa, you're gonna make one of us fold and give him better odds, waaaaa." Yeah, so? That situation is a great way to make money off of big stacks when you have a strong hand. Checking it down is collusion.
People are sore losers, and surprisingly sore winners.