Speculation: Old Biff returns to the changed 2015 (designated 2015A). Doc and Marty "transitioned" to 2015A without noticing. Why didn't they notice? Maybe because they were out on the street when it happened and Hilldale looks the same in 2015A.

BTW, time travelers "transitioning" between changed timelines is implied by Doc Brow when he tells Marty not to worry about leaving Jennifer on the porch in "alternate 1985". After they restored the timeline Jennifer was still sleeping on the porch.*

Also, from Futurpedia:

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.

 

This can be explained for two different reasons:

 

• First, these events created a time paradox. Having created an alternate timeline by giving his younger self the almanac, the old Biff from 2015 no longer existed, once 2015A became part of the 1985A timeline.

 
• Another possibility is that Lorraine, in 1996 of the alternate timeline, shot and killed Biff.

 

These events are never depicted in the trilogy, although Robert Zemeckis and Bob Gale suggest this theory as a way of explaining Biff's "disappearance."

I took some screenshots from a YouTube clip:

Speculation: Old Biff returns to the changed 2015 (designated 2015A). Doc and Marty "transitioned" to 2015A without noticing. Why didn't they notice? Maybe because they were out on the street when it happened and Hilldale looks the same in 2015A.

BTW, time travelers "transitioning" between changed timelines is implied by Doc Brow when he tells Marty not to worry about leaving Jennifer on the porch in "alternate 1985". After they restored the timeline Jennifer was still sleeping on the porch.*

Also, from Futurpedia:

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.

This can be explained for two different reasons:

• First, these events created a time paradox. Having created an alternate timeline by giving his younger self the almanac, the old Biff from 2015 no longer existed, once 2015A became part of the 1985A timeline.

• Another possibility is that Lorraine, in 1996 of the alternate timeline, shot and killed Biff.

These events are never depicted in the trilogy, although Robert Zemeckis and Bob Gale suggest this theory as a way of explaining Biff's "disappearance."

I took some screenshots from a YouTube clip:

From Futurpedia:

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.

This can be explained for two different reasons:

• First, these events created a time paradox. Having created an alternate timeline by giving his younger self the almanac, the old Biff from 2015 no longer existed, once 2015A became part of the 1985A timeline.

• Another possibility is that Lorraine, in 1996 of the alternate timeline, shot and killed Biff.

These events are never depicted in the trilogy, although Robert Zemeckis and Bob Gale suggest this theory as a way of explaining Biff's "disappearance."

I took some screenshots from a YouTube clip:

Speculation: Old Biff returns to the changed 2015 (designated 2015A). Doc and Marty "transitioned" to 2015A without noticing. Why didn't they notice? Maybe because they were out on the street when it happened and Hilldale looks the same in 2015A.

BTW, time travelers "transitioning" between changed timelines is implied by Doc Brow when he tells Marty not to worry about leaving Jennifer on the porch in "alternate 1985". After they restored the timeline Jennifer was still sleeping on the porch.*

Also, from Futurpedia:

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.

This can be explained for two different reasons:

• First, these events created a time paradox. Having created an alternate timeline by giving his younger self the almanac, the old Biff from 2015 no longer existed, once 2015A became part of the 1985A timeline.

• Another possibility is that Lorraine, in 1996 of the alternate timeline, shot and killed Biff.

These events are never depicted in the trilogy, although Robert Zemeckis and Bob Gale suggest this theory as a way of explaining Biff's "disappearance."

I took some screenshots from a YouTube clip: