The 75th Hunger Games was all a ploy to start an official uprising against the Capitol. Unfortunately, the rebels failed at rescuing Peeta from the arena and he was captured. Throughout both installments of The Hunger Games: Mockingjay, it was revealed that Peeta was physically and mentally tortured by the Capitol.

How did Peeta get brainwashed?

Beetee explains that Peeta was put through fear conditioning using tracker jacker venom. In other words, the Capitol messed him up. They stripped his identity down by beating him, shocking him and doing whatever it took to put him in a dissociative state.

What movie does Peeta get brainwashed?

It was most notably used on Peeta Mellark during the rebellion in Mockingjay. Notably, hijacking in the Hunger Games is not to be confused with hijacking as in illegally seizing a vehicle, which is what the word is used for in the real world.