When old Biff steals the time machine to change the past, he returns the Delorean back to 2015 so Doc and Marty would suspect nothing. He should instead have arrived in the alternate version of 2015 just like Marty and Doc went to the alternate 1985. No explanation is given as to why he can return to the unaltered time-line while Doc and Marty can only get to alternate versions of the time-line. In
Học Thuyết Vụ Nổ Lớn (2007), the episode
The Focus Attenuation (2014), they suggest that this could be because when Old Biff travels forward, Young Biff hasn't made his first bet that starts the time-line changing yet, thus allowing Old Biff to return to the original 2015.
Possible explanation: It is implied that Lorraine eventually got totally fed up with her abusive married life, and shot Biff to death sometime within just a few years after the time when Marty saw them in Alternate 1985 (this is the real reason that Old Biff clutches his chest and crumples to the pavement as he is exiting the DeLorean after returning to 2015, not because his aging body couldn't tolerate the physical stresses of time-warping, as most audience-members would have assumed was happening), removing him and his toxically-greedy influence on the Hill Valley area. The logical "further" idea, therefore, is that the decent-hearted Lorraine --- along with 99% of the other locals, who of course would themselves have also strongly disliked the dystopian wasteland that their community had degenerated into --- would then have banded together en masse --- "in numbers, there is strength" --- and ousted whatever "scum of the earth" officials and politically-influential residents were currently present in the area, and then made sweeping changes for the better to get Hill Valley back to being the peaceful bedroom community that it had been before Biff's interference that had started in the late '50's. Also, Hilldale --- where the McFlys were now living --- was quite a distance from Hill Valley, and so it's possible that Biff's dissipative influence hadn't reached that somewhat-geographically-removed area so much as it had affected his own immediate stomping-grounds.
