I queued for specific dungeons on the Dungeon Finder, quit right at the start (no mobs, nothing), got the 30 min lockdown, which is fair enough.
But the dungeon also appears now locked and I cannot queue for it anymore. Even though all bosses show as available.
I presume you get locked into a HC instance if maybe some other player has advanced further and engaged some mobs even in another part of the dungeon?