I have been noticing that I have been knocked down by a few destruction spells and have knocked a few enemies down with my own.
What I would like to know is what part of the spell factors into knocking someone down? Is it the AOE, or the total damage dealt, or something else?

I am hoping it is AOE so I can make a weak but wide AOE spell to knock many opponents over.

I believe it has to do with how much fatigue you have. A spell that completely drains something's fatigue will cause it to collapse. That combined with the momentum of the projectile knocks you back.

I guess I should add that I am talking about the elemental spells of fire, frost, and shock. Fatigue is not a factor in this. I merely want to know what factor I would need to increase in say, a fire damage on target spell, to knock someone over easily. It could be the blast radius, or it could be the amount of damage dealt. I know that with melee weapons the more damage done with a single swing the higher the likelihood to knock someone over.

I have knocked over whatnot with both single hit spells and AOE. But not so often so my guess is it has to do with fatigue since this often happens in combination with melee fighting. However I′m not sure.

I think it has to do with the magnitude of the spell. In addition, I think lightning spells are the most likely to knock down, followed by fire, and then frost. It happens all the time with the wizards fury spell, which is just all the elements in one no area target spell.

I've only ever seen enemies knock down (or knock back actually), when I have a damage spell that damages over time and they die a few seconds after I've cast the spell. They survive the actual impact of the spell, get knocked back, fall to the ground and die by the damage over time effect.

I'm pretty sure it has to do with fatigue, and/or paralyze. I take someone's fatigue down, then spell 'em, and watch them fly. Also, if you paralyze someone or something, you can hit them with an AOE spell, and watch them fly.

i think if the duration of the spell is enough to kill them they will fall over, but usually only for like two seconds before they die

Well, the fireball spell you get from the skingrad recommendation quest will knock over enemies, and then they die once the full 3 seconds passes. An instance where this was not a factor was when fighting that "rogue mage" for the anvil recommendation she knocked me down with a shock spell, not paralyze, and I didn't have any dent in my fatigue being that I was a mage too.

There's more than one thing going on. There's the fatigue knockdown, but there's also the physics. You can use a spell like a fireball to move objects around. My characters use them to knock welkynd stones down, or to move bodies out of ayleid traps to loot them safely.

i'm also very interested in the knock back effect. clearly, this is not based on fatigue or paralyze.
my henchmen have the spell 4fe05 in their spellbook. it does 100 points of shock, fire and frost for 10 seconds and cost 0 mana. it's in vanilla oblivion, it just makes my henchmen useful is all (as if i had taught them to spell stack). both mage henchmen and the DB hench will cast it every 2..3 spells. you can add it yourself, and notice that there are no fatigue or paralyze related effects in it, however, you'll get a knockdown approximately 50% of the time.

since i don't want to keep going back to grab henchies, i've also given them the spell 558d9. this gives them 10k life with 1k regen every second (permanently). this allows them to survive tazing each other with the above spell by accident. even when they survive tazing each other, they still get knocked down about as frequently as enemies, approximately 50%. so survival doesn't seem to play a major part in whether or not they get knocked down. however, i call it 'taze' even though it has no name because it manifests as a shock spell - the auxillary effects are not shown.

this does seem to be related to total damage. my personal attack is 651da, doing 10000 damage health for 5 seconds in 5 feet, and has NEVER KB. my guess, is that leftover damage is translated into potential knock back effect with higher affinities to shock and no affinity to damage health. this occurs even when they don't die due to regeneration or resistances - henchies tazing henchies causes them to get rag dolled into walls and then they get up a second later. test if the weak aoe does KB. it may also be related to hp loss - any single attack that does more than 50% damage, may cause this as well.

I made a 3 pts fire damage for 1 sec in 90 ft to test my AOE knockdown theory. I couldn't knock down even a rat or a deer. The spell is good for getting enemies attention and making a royal mess of someones house though lol.

I too think it has to do with AOE spells, but it seems randomly done. Once in a while an enemy goes flying, especially with Fingers of the Mountain.