There are a lot of different “groups” of these vehicles out there.
There are the licensed full-sized motorcycles that people are just driving in bike lanes and park paths at speed. Usually just one. I get passed by one almost every time I run over the Queensboro. It’s especially dangerous considering how narrow the bike paths are, and if you’re in their way they honk and yell. I’ve literally had to low-key jump out of the way to let them pass. These riders are usually not wearing helmets, are not of any particular identifiable demographic, and appear to mostly be trying to avoid traffic. I bet the only reason cars aren’t doing this is solely that they don’t fit.
There are a lot of younger people who have hacked a two-stroke gas engine to their regular bicycle. (I think someone is selling kits to do it). They are extremely dangerous to the rider and anyone near them: bicycles aren’t built to handle that kind of torque or stress. They’re also in the bike lanes and park paths. Sometimes solo clearly trying to avoid attention, but also in big group rides.
There are the ATVs and dirtbikes. I almost never see them ridden individually: it’s usually a large group ride. They’ll literally ride out into the street to block it, and then take it over to do tricks and race. They then bail off the road into a bike path or park, ride around, and re-enter the street somewhere else to do it again. Lately they’ve been gathering in parks and riding around on the lawns causing considerable damage. They’ve been mobbing the West Side bike/pedestrian path as well recently.
At night, the dirtbikes, ATVs, full-sized motorcycles, and illegally tricked-out cars race back and forth over the Queensboro bridge (and other bridges) for hours. They’re so loud the windows rattle. It’s almost every single night. I see them in the bike lane up there if I’m out for a night run.
Pre-COVID, these were all still happening, but not that commonly. Every now and then a big group ride would happen, but it was rare enough to be noteworthy. For the last several months, they’ve been increasing significantly.