The tipping point was the death of Scrapjack’s aunt/Lamington’s sister, Jaffle, which was mentioned here: >>2641977

When Strudel found her dead, it… kinda broke something in her. One day she was a loveable troublemaker who just wanted to make her friends smile, and the next she was a vengeful rebel who was bordering on completely manic at times. There was always a part of her that knew things in her life were unfair, a part that wanted to stick it to the king and start burning shit down, but she’d never really been motivated to. Until that day, of course. She just couldn’t deny her reality anymore.

After the whole ordeal, she went to Suki to try and take her mind off of it, and she immediately knew something was wrong. Strudel ended up breaking down and telling her everything—how her aunt had been murdered by a guard, how she felt like she was going crazy with how angry and sad she was, how she just wanted to tear someone apart to get it out of her system. Suki was almost as upset, as she had known Jaffle as a kind, caring mare, someone who Strudel loved dearly. She gave comfort as best she could, and eventually Strudel managed to doze off. Suki went to the kitchen for a drink or a snack, and ran into her mother, who grilled her about why Strudel was so upset. She must’ve heard her crying.

Suki simply told her that Strudel’s aunt had been killed by a royal guard.

“Well, if that’s what happened, she must have done something horrible.”

“She didn’t.”

“The royal guard protects us. If they hurt her, then they did it so that she wouldn’t hurt anypony else.”

“Or they did it because they were bored.”

“Suki.”

“They do not protect us, mother. They control us. Do you see how they look at me? Do you know how many times they have stopped me on my way home from school and tried to make me admit to something I have not done? They do not care about us. They are scum, and they should be thrown in the caves beneath Canterlot. All of them.”

What followed was a storm of Bronze defending the royal guard, and Suki finally snapping after all the shit she’d put up with from her mother and basically telling her to fuck off and get her head out of her ass. Eventually Suki stormed off to her room, grabbed Strudel—who had been woken up by the shouting—and left through the back door. That was the last time she ever spoke to her mother.

They went back to Strudel’s house, where Lam provided them with enough sweets to calm them down, and the girls started devising a plan to avenge Jaffle. It wasn’t specific to the unicorn guard who had killed her—though if they saw her along the way they’d be sure to take care of her—it was more of a large-scale plan to reform Canterlot by killing all the people who were contributing to how fucked up it was. CEO’s, government officials, politician, the king… y’know, just normal teenage girl stuff. Committing regicide and all that.

The next day Suki went back to her house and gathered her belongings while her mother was at work, and Strudel did the same while Lam baked in the kitchen. That night, they planned to head uptown and begin their reign of terror.

It might seem like Scrapjack is the driving force behind their whole endeavor, but really, Rotgut could’ve probably talked her out of it. But she didn’t, because she was just as fed up with the city. Ponies were either scared of her or hated her for how she looked, everyone thought she was an idiot because she kept her mouth shut most of the time, and her own mother was ashamed that she existed. She was looking for an out, and Strudel’s sudden heel turn was just what she needed.