For Estella it'd be a humiliating loss that is unabashedly on her. The sort of failure that you can't write off or blame on someone else. She'd probably recall her Pokemon, maybe even hide the balls away, and rage in a room until she's finally vented those emotions.

Frankly, she probably needs an experience like that to truly start to grow

For many of them, it's simply someone doing something harmful with no greater goal, motivation, or cause, just because they enjoy it.

Whilst a common one between my gals would be seeing their Pokemon hurt (like, life threatening kind of hurt) by someone or Pokemon curelty in general, for Margaret specifically it's having someone looking down on her. It ties back to her rather rough relationship with her family to which she had to consistently try and prove her worth, from which really stem quite a few of her issues. But yeah, if she feels she's being looked down on or underestimated by someone who thinks themselves higher than her it would absolutely let her emotions fly and drive her to knock them down a peg and then some.