Thanks for the replies. I did wonder if there is a limit on the number of players in each district which might explain why you are not allowed to jump up until a space becomes free. In which case trying to get back to your group can take a little time. Like others I have missed looting a boss. But there is always another chance and I guess something we just have to live with.
As written above, it is likely "instance is full" problem. What really baffles me is, that this annoying (and somethimes lethal) problem comes back with each IC event and yet there doesn't seem to be any effort to fix it, or do something to sofften the blow (like adding extra campaings for the event). Cyrodil isn't all sunshine and rainbows performance-wise either, but at least one can choose to do BG daily for 2 event tickets instead. Meanwhile the only way to get third ticket is IC (+ it's only place where style pages drop). Yes, in moment of twisted genius, someone at ZOS decided to funnel entirety of ESO playerbase into measly four campaings of IC, without any apparent adjustments. No wonder there are problems.