It's possible if you queue into a dungeon you've just cleared (without leaving the dungeon) you get put into the same instance by group finder, meaning it's already cleared.
It could be that you were matched with groups which have been doing this. So they clear the dungeon, queue at the end whilst in the dungeon, they all get put into their instance again and you pulled in.
If you haven't already though make sure to put in a bug report in game or on the forums section with as much info as you can, this means it gets put on ZOS' internal bug tracking and hopefully fixed.
You don't know how many times I've queued for a dungeon only to join and realize that it's, apparently, already been 'completed', so we're all just standing around like idiots, wondering what's going on. And if you leave you have to wait until you can queue again. This recently became a problem, too.
(Xbox One - NA server)