Yeah, that is some bullshit right there.

I used to do big events, and without fail, I got a schedule of fees to be paid leading up to and throughout the event. Every single hotel I ever worked with did this - the dates and percentages are in the contract. No chain hotel is so new to the hotel conference business that they don't have this down pat.

I would bet that part of what happened here is that the con didn't meet the guarantee for hotel rooms. That would explain a bigger payment than they would have expected being due mid-event, and also people being threatened with getting kicked out of their hotel rooms. They probably expected those rooms to be comped and when the guarantee wasn't met, they got fewer (or no) comps and organizers and speakers had to pay. Of course, when attendance didn't meet expectations, they should have known this was going to happen so it doesn't excuse their mismanagement.