Lag: Not worth staying around, win or lose.
Lack of Confidence in Their Character: I had this happen a few times today. If you two stock them the first match, they're probably afraid it will happen again, so they leave in hopes to find someone on their level or someone they can actually beat with ease.
2 Stocked: If you're the one who two stocked and you know you're a better player, should you stay knowing you'll win again, or find someone around your level or one that has you on your toes? If you're the one getting two stocked, same deal. It's elitist, yes, but some people want to have a challenge. And yes, in a way, it's ironic, because the person you two stocked may be looking for a challenge him or herself.
Boring: The opponent thought you were playing predictable, heavily rolling, or too patient, that it got to the point of no longer caring and didn't want to play you again because you were being boring in their eyes.
Cockiness: Whether you're the one winning or losing, do you want to deal with a cocky player? It's a lot more satisfying to take out a cocky player who taunts at your stock loss and then leave after, because they're left feeling salty.
Had To Leave: Maybe they had time for one more match, backed out from one room and went into another, and you were just the last person they were going to play for the day or that time.