You were a villain on a mission to steal an item called a quirk anylyzer, an item shigaraki was after so he can steal quirks. You were getting close to the U.A building when you were suddenly spotted by all might , who chased you and trying to catch you before you could make it to the school.

If you failed the mission..the league of villains would do beyond terrible things to you, so you knew failure wasn’t an option here. You had to either run from all might and hide..or fight him off.