After arriving back with his family in 1991, Doc opened the Institute of Future Technology and soon after, went back to 2015 and bought the almanac. Marty races back into town and finds the Doc of 1955, who had just sent the original Marty back to 1985 seconds earlier at the courthouse. The first perspective of 2015 that we get is Doc's, whose presence in 2015 causes yet another timeline to branch off.

Biff, just as mean as he was in 1955, now a ruthless enforcer for the Tannen Crime Family in an alternate 1986. In scenes deleted from the film, Biff Tannen of 2015 fades out of existence (much like Marty was in the original movie when interfering with his parents' meeting) once arriving in the future, having been erased from existence in that time period.

The short answer is that his pain was caused by having changed the future enough to erase his own existence. Though they know a lot about each other, Biff and Emmett Brown are never seen interacting in the films — only in the animated series and ride. While normally maintaining an appearance of being obsequious, Biff's original bad attitude still lurked beneath the surface and emerges occasionally, like his reaction to seeing the DeLorean take off and when he snapped at Marty when Marty approached his truck after his adventure in 1885. Even if he had jumped in the time machine and traveled back to 1985 Hill Valley he would have had to wait a week. The Grays Sports Almanac had been released on December 31, 2000, marking the end of the 20th century.

Eventually young Biff used the sports almanac to bet on the results of sporting events, since he knew the outcomes in advance, giving him the ability to cheat.The dust jacket was gone, the pages were worn and turning yellow, there were even a couple of what looked like mustard stains on the spine. He also warns his younger self that at some point in the future, a "kid" or a "crazy, wild-eyed old man who claims to be a scientist" may show up asking about the book. It is my reasoning that once he landed again in 2015, the changes he made started to solidify and as Marty and Doc enters the DeLorean, the new timeline is completed, thus removing Old Biff from existence. On October 26, 1985, Doctor Emmett Brown (Christopher Lloyd) arrives from the future and tells Marty McFly (Michael J Fox) and girlfriend, Jennifer Parker (Elizabeth Shue), he needs help to save their future children from getting into serious trouble.

I got my hands on a DVD, the same deleted scene is there in it with a commentary that says that they intended it to be that Lorraine does shoot him at some point in time. Using it as a guide for betting, by 1985 he became a celebrity and built a financial corrupted empire in an alternate reality, where he secretly murdered George McFly, married Lorraine Baines McFly and ousted their son, Marty.Eventually, Marty and Jennifer get married in the Chapel O' Love and have two children, Marlene and Marty Junior. No sooner has Marty arrived back in 1985 than a modern train comes through, smashing the DeLorean to bits while Marty jumps to safety. Biff angrily tried to clobber Marty, only to in turn get hit over the head from his father's old transforming speakeasy table and knocked out cold. When Doc and Marty travel back to 1985, they run headlong into the ripple effect and end up in the alternate timeline. The driver of the car turns out to be Lorraine's father, who — not realizing that George was peeping through his daughter's window — brings George into their house to recuperate.