While the initial surveillance was to stop Trump and help Clinton, Obama used FISA to provide a “national security” cover for politically spying on Trump right up to the inauguration.
The first 2016 application was made the month after Trump obtained the nomination and the second in October, the month before the election.
As the unmasking picked up pace after the election, the reasonable assumption is that its purpose was to undo the results of the election or hamstring the incoming President.
Now Obama and his allies are or should be terrified that the scope of the illegal surveillance is revealing their criminal acts.
This is why I believe Mueller is growing increasingly desperate to find one crime by one person he can force by threat of jail to provide any shred of anything that might be used to justify their illegal espionage.