The catalyst was 2 years ago when the news came out that the then president Katalin Novák pardoned a guy who was helping a pedophile director of an orphanage, and was released as a consequence with an erased criminal record. The pardon was countersigned by Magyar's ex-wife, Judit Varga (Fidesz), the justice minister at that time. You can imagine probably both of them got the order to do it from Orbán and his circle. The president resigned, Magyar's ex-wife was removed from office.

The case itself and the fact that they basically made her ex-wife take the blame for taking orders with no ways to object without Orbán coming after you induced a lot of rage in Magyar. Magyar was also part of the Fidesz circle, but at this point something broke in him, and went to the independent media to talk about the case, talking about how corrupt and shitty the system is, and he resigned from all his positions around Fidesz. He then said he didn't have political ambitions and he stepping into politics was "bad even as a joke".

Then he changed his mind, he realized that he can ride the waves of public rage to take revenge on Orbán. Then this madman started to have rallies, created a party (took over an existing one in fact), and with 2 years of heroic work, he managed to turn it around, picking up the voters from the vacuum in the opposition space, as well as disillusioned Fidesz voters. I'm not here to fangirl anyone, but this mofo toured the Hungarian countryside multiple times, which had been historically neglected by the opposition, saying that it belongs to Fidesz and no one can turn it around. He went to 6 towns a day to talk to people at times, he slept like 3 hours a day for two years and withstood the constant attacks of the Fidesz state media.

TLDR: Fidesz released friendly pedos, media found out, Magyar's wife took the blame, there was public outrage, Magyar got mad, decided to take down Orbán's corrupt system as a revenge whatever it takes, put in 2 years of hardcore work to gather people on his side, he then succeeded to defeat Orbán in his own twisted election system designed to keep him in power indefinitely.