What exactly causes mobs to ball up?

I flag my henchmen away and run towards a mob. Sometimes they stop attacking me and ball up to a single location while other times they don't.

Just want to know how can I make it so that the latter doesn't happen anymore and that they ball up all the time.

Also, is this strategy viable in high-end PvE?

Depends on how close the rest of your party are (ie, if they're in the enemy's own aggro bubble) and what type of weapon and HP levels everyone has (the aggro mechanics prioritize caster weapons and having lower health, with lower armor & movement speed acting as tie-breakers).

Balling (and keeping them balled) is usually accomplished by using obstacles to your advantage. A minion army can often get foes balled up, but choke points and corners are probably the most reliable (though you can't always maneuver to one in every fight).

http://www.youtube.com/watch?v=A8NgsqRolAk

May be of interest.

PS: To address your last question, yes, this is viable (balling at least; I don't usually put as much effort in as is shown in that video). In fact, you'll want to do it as much as possible so that you can get as much out of RoJ/Splinter Weapon/Shatter Hex/etc as possible.

I think I found this on LOD page once, can't really remember.

Aggro 101:
When another person is in aggro, casting on them will give you aggro.
When another person is in aggro, attacking a spirit cast by something they have aggroed will give you aggro (but only the group that cast it).
When another person is in aggro, touching what they have aggroed will give you aggro.

When another person is in aggro and you get aggro from them, you must die. You can run 200 compass ranges away from that person but you will still have aggro and as soon as you return to the aggro's compass range they will continue following you.
The exclusion to this scenario is that you have escaped aggro radius (though you still technically have aggro), but the person who is holding it dies, all aggro is erased.

Examples:
Person A is the tank and Person B is the healer.
Person A is balling aggro and Person B casts a spell on them, Person B has aggro.
Person B runs away and Person A dies. Person B no longer has aggro.

Now let's add a third person.

If Person B runs away, but Person C, our trusty spiker, thinks to themselves, "Hey, lets try and kill the aggro!" and they get aggro themselves, Person A and Person C both need to die before Person B will lose aggro.
If Person A dies while Person C is spiking, Person B still has aggro. If Person C tries running away after Person A dies (and is surviving), Person B still has aggro.

These same principles work for NPCs. If Yendzarsh casts Energy Burn on a spirit as it is running past, Yendzarsh has aggro, and will continue maintaining aggro until:
A) the person who had their aggro first dies and Yendzarsh outruns it (the most unfavorable situation, most prone to failure)
OR
B) the tank breaks the aggro onto the rest of the team (aggro is more attracted to squishy Mesmer and NPC than tank, even though tank still technically has aggro) and the aggro is killed

Also the bodyblocking for dummies video EFGJack made is always a good resource, but the foes he chose to make the video with aren't all that hard to ball up in the first place.

If you run into/past a mob with a speedboost up, depending on if the mob has any kind of speedboost itself, they will just all run back to a single dot. It's how I ball 1st room foundry and some other places in gw with minimal effort. A cupcake or fallback should do well enough to ball them up like that, but I wouldnt use it as a reliable balling method as they normally break apart as soon as you stop.

Basically, balling them up in order to tank, in GW1, can be done, but not like it can in some other games, because of the AI. While it can be done, and is therefore, 'viable', I don't think it's really worth the effort. It's easier just to fight within the 'rules' of the AI than to try to force the AI into your method.