Currently with the badly designed system there is a cap on the number of parties that can be inside of any particular dungeon at a time. That wouldn’t be too much of a problem except that this number caps out really fast. What this means is high level chars at the end of their day making a 1 person party and griefing lower leveled players by afking inside of a dungeon. No timer means 1 slot can be taken up indefinitely.
tl;dr
Either remove the instance party cap or implement a timer to stop griefers.