The one Trump rally I attended, there was no reason to bring chairs and camp out because the venue was large, the weather was decent. This venue in NJ had 4 times as many people wanting to attend than the size of venue.
So they came 24-48 hours ahead of the rally to wait in line, in the cold NJ weather. So they had to being a chair and a blanket. Then they were told they could not bring those inside the building.
So what would you do? Take your chair home and miss the rally after waiting 2 days and 2 nights in line? Nope, most just abandoned their items.
I think your explanation explains it. I took a second look at the pics and sure enough, the trash is full of collapsible chairs. People wouldn’t normally ditch those chairs like that. What apparently happened is just what you said — they unexpectedly had to leave the chair behind and probably a few trash items with it, both of which they had probably expected to carry out with them. Can’t say for sure, but my guess is despite these pics looking pretty bad, it would have been far worse had it been a leftist rally of some sort.