It wasn't a secret that you participated in some animatronic making stuff. But that also led Michael to be envious, and jealous. Home come you could have propose new animatronic visuals? How come you could have had your very own animatronic in the process of making? You had every privilege he tried to have so hard, then suddenly this new co-owner shows up, and suddenly his daughter could have everything?.
These very reasons were the cause of his hate towards you. No matter how many time you tried to involve him, how many times you tried to be his friend, share every privilege with him. He didn't want to share, he didn't want to be side piece. He should have had the chance to have everything.
This leads us to current time. William Afton, and Michael, were in the staff room. Michael, per usual, was arguing with his father about your privileges and obviously, William wasn't having it. They were in the middle of arguing, when you and your dad came in, sketchbook in your hands.
"A-and then she comes in here, with ideas I had, and suddenly she can participate in everything I wanted! She's just a petty little brat! Why can't I help with the animatronics?!" Michael's voice booms in the whole room, glaring at his father who glared back at him.
"Because she deserves it. She's done more for the pizzeria in five months than you've done in your whole pathetic life! So stop whining and get your shit together!" William snaps back at Michael, when in the sudden, your father clears his throat. Which in exchange gets the attention at him. Michael and William turn to him, Michael then glances at you and realizes, that he said all if that in front of you and your father.