Understandably there might be circumstances that the person has no control over, ie ISP d/c or Err=7 or some real life situation they need to take care of, but the majority of cases I have encountered or read / heard about usually fit into these descriptions I made up over some dwarven ale

Fear of Death.
Enchantment spell. For duration of mission - on death, quit.
A simple one, they die they quit. They seem to have this god syndrome for their character where dying is not acceptable in their characters world.
Fear of Failure.
Hex Spell. Causes target ally to quit when party loses x player or is less than full.
This is a cascade of quitting due to the belief that certain missions cannot be finished without X character or a full party. They might be a real good party mate but as soon as the party becomes 7/8 they vanish. This has an unfortunate side effect of causing other members to catch this disease too.
Suicidal Buffoon.
Stance. For duration of mission act like a strategical nightmare, when things look bad throw a tantrum and quit.
Usually a suicidal mob training egotistical character who has "done this a 1000 times" runs off at inoportune moments, thinks the rest of his/her team are newbs (or noobs or n00bs depending on how elite, l33t or 1337 they feel at the time) and then die because they fail to realise that a team based game isnt about 1 person, they yell at you then quit.
Griefer.
Preparation. For duration of mission you destroy the party, when your satiated you quit.
They train mobs or do awful stuff detrimental to the party, insult you then quit. They might even hold you to ransom.
Impatient.
Trap. For x seconds (where x is time till you have to leave) you try to rush and hurry, when x seconds is up, Impatient trap goes off and you quit.
Since they didn't adequately allocate time to do the mission, they are constantly rushing and telling the team to "HURRY UP!", when the team cant... they guess

"This Way!".
Shout. Some allies head your call and listen to you, the others do not.
The learned one, usually has a good idea about the mission and great idea's about teamwork and strategy. Usually a leader or so. But then there is the rest of the team... a right bunch of knuckleheads. Usually pm's the good ones, quits then heads back to town with a select few forms a new party and tries again.
Feel free to add more or pick the one which you fit into
