I had seen somewhere on the Internet where some bullets coating would react with powder - I forget the details.
I've had powder go off due to poor storage conditions - exposed to a hot wall repeatedly over many years, but it typically corroded through the case at the mouth and/or the primer. Your's looks to be corroding at the base of the projectile where powder meets coating.
Yes, the possibilities include breakdown products from a deteriorating propellant (which typically include nitric acid) collecting there near the base of the bullet because of the direction the rounds face in storage, and attacking preferentially in the crevice.
It doesn't look like an external cause.
