Hard to say exactly what's the cause without more details, but the 8's show up when there is a required field for a system-driven transaction that cannot be populated. So the system had inventory to put somewhere, but it couldn't find the location for where it was supposed to go so it made one up.

This kind of thing is usually more common with MOs backflushing lot controlled materials that haven't been issued yet, resulting in a negative balance of 8888888888 lot #.

In my experience, those are created when you have a back flushed item (MMS002 issue method) without a default location assigned (also in MMS002).