That's a good question. I don't know, but:
Sometimes the motorcycle officers will race ahead to the intersection so they can dismount if necessary to direct traffic.
Then they must race again to catch up and pass the motorcade to beat the lead auto to the next intersection.
Another pertinent question is; how much experience did this officer have?
Was she/he cut off by the motorcade?
Was the bike inspected recently, and in good repair?
Where was Hillary when all this happened?
Where was Bill?
Where was McCauliffe?
Exactly. If you have ever been in a funeral procession which is escorted by two police cars or motorcycles, they do the "leap frog" thing where one stops traffic at an intersection and the other replaces him as the procession goes by. They have to speed by the procession to get to the next intersection.