Members will switch gyms if it's too busy at times they want to visit. "Too busy" includes too much contention for a single piece of equipment.

US gyms might be vast warehouses but in the UK, most only have a couple of benches, couple of cages, one set of db per denomination above 20kg etc. They require working-in and consideration for others.

A couple of unapproachable "heavy users" doing 3 hour sessions across peak hours can ruin the workout for dozens of paying members needing a few min per station for ~5 sets.

It might also be a euphemism for "dickhead" who also tend to be "heavy users". Those that damage, hoard and don't share equipment and repel other customers on many levels besides - threatening, lecherous, loud and smelly.

Doesn't even need malicious intent - can be weirdo bores, forever talking at victims while doing a routine that makes absolutely no sense besides camping on equipment for half a day... 100 sets of incline press 7 days a week... what are you even doing to yourself fella?