first off let me say that sunder ROCKS!!!!!.......i truly love this spell.......i also fall down when i use it and why i think this is because sunder is a crazed bad *** ae swing.......imagine if you yourself were facing five foes and wanted to try and hit them all at once............most likely you would fall down after you hit them cuz you woulda lost your balance..........i believe this is the reason you fall down. now as far as your enemy falling down.....i noticed if i use sunder and other people are using alot of spells in my group at the same time the fall graphic maybe skipped due to too much content, not sure on this........but i have noticed that the mobs that are in a certain range will only fall down those out of melee range wont fall but they still get hate for you do to the fact sunder even when hitting one mob gives hate from all in encounter which is a great agro booster.
i love using this when ae space is available when grouping.........i'll go out and pull mob.........on my way bact to group ill cast bloodrage on myself and when i reach the group i turn and use sunder and my ae swing....by this time i got so much agro going that even if the healer has to go crazy with the heals due to a long pull or somthing he never gets agro from me.......sunder....use it ......love it