Electrical gremlins

What amp fuse is inline?
Volts don't mean squat really when it comes to fuses, amps do and if you are running less than a 30 it doesn't surprise me that it blows.
Main fuses in some cars are 50 and 60 amp.

I think what is happening is when you start the car it draws the battery down a little and when the car starts it tries to replace that used power from the battery and blows the fuse because it's too much amps for the fuse to carry.
ESPECIALLY if it usually only blows on startup and not any other time.