It could be celluloid rot. Celluloid, (cellulose nitrate, which is similar to gunpowder in chemical makeup) if it deteriorates, emits a highly corrosive gas that will rust razors very badly, indeed. If this is the case, you'll need to remove the scales and discard them before it primes other celluloid handles to break down, too.

if there's discoloration in the handle that is in the same areas as the accellerated rusting, it's a sure sign. If the celluloid smell is particularly strong on the razor, that's also a sure sign.