Nerds and men with wizard beards near loads of young women, almost all of whom are still in high school. Men with lower than average social skills, grooming and attendance to the normal rituals of social behaviour. American high school stuff, basically.

The issue at that convention center (from my experiences there) are that multiple events really occupy the same space, not adjacent spaces. Like if there are 4 event halls in a row with two groups (A and B) it won't be AAAB, it'll be ABAA. And you have to travel through the common (very large) hallway to get to these areas.

I could understand parents being somewhat disconcerted with finding their teenage daughters surrounded by a bunch of adult men. Especially if they didn't know that another convention was going on (at first). Imagine you take a large group of young teenage girls somewhere and find that there are a couple thousand adult men seemingly at the same event. At first it'd seem very odd.