I’m home sick from work today, so I’ll give it a shot… I’m curious to know the ins and outs of it as well. If anyone can add to this or even debunk it, please do! 
First we have to look at the two types of falling that occur when you hit someone out of (or into) the air:
**Reset **- they land on their feet
**Knockdown **- they land on their back
You can’t juggle after you hit them with a move that causes reset. So knockdown attacks are the important moves that set up juggles.
There are two types of knockdown attacks:
hard knockdown attack - will knockdown a grounded opponent
soft knockdown attack - will only knockdown an airborne opponent
A general Street Fighter rule of thumb is that all special moves will cause knockdown. If not Hard knockdowns, they will at least be Soft knockdowns. Normal attacks on the other hand (with the exception of sweeps) do not knockdown and will cause a reset against airborne opponents. There are exceptions, especially with command normals, but for the most part this is the case.
Next, you have a few different knockdown states that the oponent can be put into:
sweep knockdown - duh (I don’t think you can juggle after this, but I haven’t tested much)
standard knockdown - only moves with juggle potential can juggle during this time
float knockdown - anything can hit them, does not need juggle potential (this is rare!)
What is juggle potential? A value assigned to moves to allow them to hit opponents who are in standard knockdown state. If you exceed this value while juggling, this move will no longer be able to juggle the opponent (you have surpased its juggle potential).
So now you just need to know which attributes and values have been assigned to which moves.
Let’s look at Sagat’s moves:
Move
- Hard/Soft, Juggle potential (Float?)
High Tiger
- Soft, 0
**Low Tiger **
- Soft, 1
High EX Tiger
- Hard, 2
**Low EX Tiger **
- Hard, 2
**Tiger Uppercut **
- Hard, 0
EX Tiger Uppercut
1st Part - Hard, 0
2nd Part - Hard, 4
**Tiger Knee **
1st Part - Soft, 0
2nd Part - Hard, 1
3rd Part - Hard, 1 (0 if 2nd part hits)
EX Tiger Knee
1st Part - Soft, 0
2nd Part - Hard, 1
3rd part - Hard, 1
**Towards+Roundhouse **
- Soft, 1
Super
1st Part (knee) - Soft, Unlimited
2nd Part (uppercut) - Hard, Unlimited
Ultra
1st Part (knee) - Soft, Unlimited
2nd Part (uppercut) - Hard, Unlimited
Focus
lvl1 - Soft, 0 (crumple grounded opponents on counter-hit)
lvl2 - Hard, 0 (crumple grounded opponents, Float if first knockdown vs airborne opponents)
lvl3 - Hard, 0 (crumple grounded opponents, Float if first knockdown vs airborne opponents)
**Focus Crumple
1st part - the opponent is considered to be standing
2nd part - the opponent is in an airborne Float state
All of Sagat’s other moves (besides sweep) won’t knockdown a grounded opponent, and will reset airborne opponents. The moves above are his knockdown moves.
Let’s run through a combo:
Tiger Uppercut (causes standard knockdown)
FADC
Towards+Roundhouse (juggle potential = 1)
1-hit EX Low Tiger (juggle potential = 2)
So, you land a Tiger Uppercut, to put them into standard knockdown state. Now you can only juggle with a move that has juggle potential. So you do Towards+ Roundhouse for 1 hit. This has a juggle potential of 1, so it can hit opponents who are in standard knockdown state. However, you can now only continue to juggle if you use a move with a juggle potential of 2 or greater. So you then do EX Low Tiger which has a juggle potential value of 2, to score 1 hit (You used up one of it’s potential juggle hits with the Towards+Roundhouse, 2 - 1 = 1). Now you have used up 2 points of juggle potential total. If you want to continue juggling you will have to use a move with a juggle potential value of 3 or greater. So the only moves that will juggle at this point are EX-Uppercut, or Super/Ultra.
Now let’s look at Ryu’s knockdown moves:
**Fireball **
- Soft, 0
**EX Fireball **
- Hard, 2
**Hurricane Kick **
- Hard, 0
EX Hurricane Kick
1st to 4th Kicks - Soft, 5
5th Kick - Hard, 5
**Air Hurricane Kick **
- Hard, 0
Air EX Hurricane Kick
1st Kick - Hard, 0
2nd to 5th Kicks - Hard, 5
**Dragon Punch **
- Hard, 0
EX Dragon Punch
1st Part - Hard, 0
2nd Part - Hard, 1
Jump MP
1st Part - Soft, 0 (Float!)
2nd Part - Soft, 2
**Super **
1st to 4th hits - Soft, 5
5th hit - Hard, 5 (causes sweep knockdown against grounded opponents)
Ultra
1st to 7th hits - Soft, 8
8th hit - Hard, 8 (causes sweep knockdown against grounded opponents)
Focus
lvl1 - Soft, 0 (crumple grounded opponents on counter-hit)
lvl2 - Hard, 0 (crumple grounded opponents, Float if first knockdown vs airborne opponents)
lvl3 - Hard, 0 (crumple grounded opponents, Float if first knockdown vs airborne opponents)
**Focus Crumple
1st part - the opponent is considered to be standing
2nd part - the opponent is in an airborne Float state
Looking at the above information, I can tell that the following combo is possible:
Anti-Air lvl3 Focus (causes Float)
Fireball (causes standard knockdown)
2-hit EX Fireball (juggle potential = 2 during standard knockdown)
6-hit Ultra (juggle potential = 8 during standard knockdown)
…and I just tried it in training mode, and yep, it works.
Overall, this system seems to help explain a lot of the juggles in this game… you just have to determine what properties are assigned to what move. Some are pretty specific and exceptional (like Ken’s counter-hit fierce DP), but besides that, once you work out the values and states through experimentation the results are very stable.
I also realize that I could be wrong on all counts, this is just what I’ve noticed so far… perhaps there is a deeper system at work here (which could explain for all the property variation) but this gets the job done for me. 