Os escritores dos filmes Back to the Future (Escritor / Produtor Bob Gale e Escritor / Diretor Robert Zemeckis) abordaram especificamente este ponto em um " FAQ Oficial " no site da BTTF:
1.19: Doc Brown of 1955 learns a lot about the future from Marty. Shouldn't the Doc of 1985 remember all of those things that happened in 1955?
A: 3 possible answers, all credible. 1) The "Ripple Effect" of time travel (which caused all of the photographs to change) does not affect human memory. 2) The 1955 Doc suffered a memory loss sometime after his adventures with Marty (maybe it was from the drugs he took in the 60's as Reverend Jim!). 3) Doc actually did remember everything, but he still did all the same things he "remembered" because he didn't want to risk disrupting the space-time continuum.
There's a 4th possibility which depends on your view of time travel. There's a theory (we like to call it the "Self-Preservation Instinct of the Space-Time Continuum Theory") that says that the continuum is always trying to keep itself "on course," and when things happen to change it, it always tries to correct itself. It is much like a river, which tries to keep its overall course. Although earthquakes, fallen trees, floods, or other circumstances might disrupt it at points, the river would cut a new channel so that it would end up back at the same place. Thus, the overall physics (or metaphysics) of the space-time continuum would insure that any of Doc's memories of events that might create paradoxes would become hazy — or be erased.