I participated in a couple Hyjal trash-only raid groups (two different groups back to back), both of which never killed any bosses - just a few waves and died to reset the waves back down to the beginning.
first group was without any guildees, second group had ~5 guildees in it. (none of them were saved and locked out of running hyjal. - it appears the first group locked trash raid only pugs long after they left that raid.)
After finished with the 2nd trash raid, went to shatt and Logged out for a few hours until raid time, logged back in for raid, and I was unable to zone into Hyjal, chat prompt said I already had a Hyjal raid ID and could not enter this raid. Checked my social tab’s Raid Info, and I had a Hyjal Raid Lockout ID number. When trying to enter the raid in a raid group by myself, it brought up a prompt saying this Hyjal instance was cleared, all 5/5 bosses were dead. - then I got a 1 minute force kick timer for not being in the group. (despite the entire raid being cleared and empty.)
It seems I may have got locked to the “first” hyjal trash run i went into, after switching groups and joining a separate instance ID.