i could say first (even if this doesnt make any sense to you): bad luck, something like a 2 seconds bad connexion lag when uber meph appears and shoots you or attacks you and with his aura etc etc etc...
then i think well, bad gear i guess, ignorance about the proper gear needed to face ubers or something like that (or people trying to kill them using "potential gg builds" that dont actually work)
and lastly they dont know "the strategy" to clean uber tristram until becoming "experts", so they try to face all 3 uber bosses at the same time without knowing how to do so and etc etc
i am a very carefull player when it comes to uber bosses... i like to be very strategic even if im using gg gear (i like to beat my own time records) and i genuinely enjoy doing ubers.
ive been called to help sometimes to people's games and as soon as i enter uber tristram the game (for real) crashed because of all the skeletons and monsters casted by bosses lol i still have some pics, but you cannot fully appreciate the amount of mobs ingame xD (those were the only games i couldnt clean, but just because my old laptop couldnt handle the pressure lmao)
but i guess ubers are still a tough enemy for some players