particularly the front its caused by the bi metal that is used in the brake pads having a galvanic effect with the tin on the metal spokes.

its because some bean counter somewhere said...

'Oi we can save $10 a bike by using inferior Italian junk'

rather than doing it properly: ie XT's build from 1965 - 2004.

Yamaha doesn't do warranty on it either.

BUT don't get me started. I am eventually going to replace all mine. JTR in Australia does spoke sets to fix the problem.