Sometimes, it's the result of a misunderstanding, or the trainer thinks that the Pokemon would be better off without them. Even the Pokemon themselves might be the ones who run away for similar reasons...but, then there are some trainers who abandon them because the Pokemon doesn't fit their ideal image of "strength" somehow, so they cast those Pokemon out and leave them to fend for themselves.

[Cilan doesn't like admitting that those sort of trainers do exist, what with him being the kind of guy who tries to believe the best in everyone, and he had even been willing to hear out that one kid who abandoned Tepig, but...by the end of the day, even he had to admit that kid was just cruel.]

Thankfully, those kind of trainers are rare, but unfortunately this sort of thing does happen. That's why, as both a Gym Leader and Pokemon Connoisseur, I have to try to help teach trainers how to properly raise their Pokemon so that there are no misunderstandings that could lead to things like that taking place.