Easy. The same reason women leave men for no reason.....they met someone else they think is better.
This whole TV sitcom-Chicken Noodle Soup for the Soul-I just need space bullsh#t is exactly that.....bullsh#t. People leave Person A to get with Person B because they think Person B is a better choice for whatever reason. It doesn't even have anything to do with Person A or what they are or aren't doing.....they just believe Person B would be better, so they leave even if there's nothing overwhelmingly wrong in the first situation. And in many cases, they find out the grass is not greener on the other lawn.
If you want greener grass, water your own lawn.
This is why you should never put anything past someone because at the end of the day, the dating game is cop and blow....because you never know when they will go. You cop someone and then they blow. There's nothing you can do about it....there's no formula to stop it.....there's no definite person that will or won't blow. People are feeble, gullible and selfish, so noone is above getting left by someone they're dating....even for no apparent reason at all.
But as Rosebudd said, if you're chick leaves and you know why, then you weren't on game. If a chick leaves you, then it should be a surprise because if you knew there was a problem, you should have got her mind right.
Peace