Anything from stone chips to badly painted from new, also maybe the wheel have been cleaned with a wheel cleaner, it could be salt, so many reasons.
It only needs to get a little moisture trapped in the hole and the chemical reaction starts.
The Alloys on my SL are fine on the outer side but the paint is blisterng on the inside of the wheels, so in the next few days I will take them off and respray the inside section light grey.
The flip side or inside the tyre? If inside how do you take the tyre out other than asking the tyre dealer?
The flip side I washed it every spring. Forever dirty and impossible to make it squeaky clean. Funny it plastic where the bolt holes are.