I'm sure some people click accidentally when that box pops up if they are the last left at the end of an instance. It seems easy to do.
I have been in the position many times where I've joined a group in progress which isn't a group in progress and I arrive only to hear the victory music. I believe it's often a mistake but some people seem to delight in maliciously opening the group this way and laugh before leaving you there.
It's very frustrating when you have queued for ages and have to re queue. I imagine this is a simple fix so that you cannot open the party to DF once the final boss is dead, could the devs please implement this?