u/spongemonkey2004

old biff being able to go back to 1955 then back to his 2015 has always been a debate but i have a new theory on this and before i thought of this new theory i have always just considered it a timeline flaw in the movie. my new theory is doc keeps warning about the implications of meeting your future or past self and how it could destroy the fabric of reality. i think when jennifer came face to face with her future self it did just that, the universe couldnt allow this so it created a scenario where old biff steals the delorean to erase the future jennifer from existence and allowed biff to return so he would also be erased and give marty and doc the setup to erase present jennifer as well who got left in the president biff timeline. Young biff was spared getting erased because he didnt know that was his future self and same with the doc not looking directly at his past self to avoid the same situation.