What happens is: there's too many moving things in one area. So say everyone is right in one area and they have engines running, animals, golems, automatic mining carts; the server has trouble processing all that info all at once. Especially when someone leaves on a tier 5 spawner and it drops 5 mobs every 2 seconds and the spawner is right next to engines and all that other stuff.
It takes a lot to make the server crash so whatever it was, someone was doing something they shouldn't be doing.