This happened to my brother the other day. Thing is, he is a Vanguard and doesn't have any kind of leap or charge attack yet. We really have no idea how it happened.
The boss can knock you down. If you are on the grill when that happens, there's a chance that you'll end up falling through and being stuck below. The best solution now is to pull the boss back to solid ground. Just move back when you engage him and fight him where you don't have a second layer below for your character to bug out to.