Offset glitch. Grant has a slightly different hitbox than trevor, so swapping characters at a certain point on the stairs puts him slightly apart from them, which puts him apart from the flat ground at the top. He bypasses the flat ground, and even though there's no stair graphic there, the game's like 'well the flat ground wasn't touched, so he must be on stairs still'. And there you go. Infinite stairway to heaven. :)