It is a topic of regular discussion between Back to the Future fanatics about the logistics of how the Biff Tannen of 2015 made it back to his original timeline in Back to the Future 2. However, there is an explanation for this seemingly impossible feat... But not in the films.
*SPOILERS BELOW FOR THE BACK TO THE FUTURE UNIVERSE*
Back to the Future: The Game, released in 2010 by Telltale Games, is the answer to this increasingly debated question. When originally watching Back to the Future 2, it seems impossible that Biff managed to return to his original timeline in order to give the DeLorean back to Doc and Marty. This is because he changed the past when he returned to 1955 and therefore he should have returned to the 'Pleasure Paradise' timeline (the timeline where young Biff became a millionaire). At least that is the common opinion. However, in the final episode of Back to the Future the game, the 'million dollar' question is finally answered. In the game, Edna Strickland (Vice-Principal Strickland's older sister) hijacks a DeLorean and travels back to 1876, where she destroys Hill Valley. Simultaneous with her departure back to 1876, Doc Brown returns to 1931 to visit Marty (due to other events that recently occurred). However, time does not immediately change after Edna is transported to 1876. This introduces a new phenomenon, addressed by Emmett as 'time-ripples'
The idea behind these ripples is quite simple. The theory is that any event that is changed in the past takes some time to come into effect in the future. This is evident throughout all of the three movies. An example of this can be seen during the Enchantment Under the Sea Dance, as Marty slowly fades away (after George is confronted by a bully), rather than instantly disappearing. The time it takes for Marty to disappear is around 50 seconds (from when Lorraine is taken away to when the couple kiss), which means that the time ripples took a time of about 50 seconds to ripple through 13 years to Marty's birth in 1968. In comparison, Biff traveled forward 60 years back to 2015. If it is assumed that it takes approximately 0.06 of a minute (or 4 seconds for the ripples to travel through time), it would mean that Biff had around 4 minutes to get into the DeLorean and safely get back to the original timeline. This example may be slightly unrealistic, as it is unknown where Biff parked the DeLorean and therefore how long it would take for him to reach it.
However, there is another variable in this scene that could affect the outcome, as Marty is dealing with his existence, while Biff is simply dealing with a change in personality. Therefore, the effects on Marty may have occurred a lot earlier than 1968. For example, in the situation where George punched Biff but did not take Lorraine back from the bully, Biff may have found and even potentially killed George, therefore preventing Marty's birth and greatly extending the time it takes for the ripples to spread through time. This would give Biff a greater amount of time to get back to 2015A.
Another piece of evidence that could contribute to this question once again relates to the scene in the fifth episode of Back to the Future the Game. When Marty asks Doc how much time they have (from 1931) to change Edna's past and therefore fix the future, he says they have around an hour (for the ripples to travel 54 years). This presents another issue about Doc's family line (which could be discussed in another post) but for the purpose of this, it will be assumed that Doc and Marty's timelines will not have been changed up until the night where Doc shows Marty the DeLorean in 1985. This means that Biff would have a comfortable time of more than an hour to get back to 2015A, which would have made it very plausible for not only him to arrive back in 2015 before the time-ripples caught up, but also for Doc, Jennifer and Marty to leave the period.
The evidence above has demonstrated how Biff Tannen was plausibly able to return to 2015A in Back to the Future 2 instead of returning to a possibly horrific 2015B, still potentially run by the Tannen family! I'm sure there are some people who have a counter argument to this, so feel free to post a reply on your thoughts about this question!