Prediction: Cory Booker will be the Democrat Candidate

There is a woman who rarely does prophecy but when she does, she is 10% correct.

So how would this be possible when Cory Booker dropped out?

Simple...

Sanders wins the nomination and appoints Cory Booker as his running mate. This would make sense as their political opinions are not incompatible and it stops it from looking like an all white ticket.

Then, Sanders dies... or, at least, is incapacitated too late in the campaign to be wholesale replaced, so Cory Booker takes over, appointing his own running mate.