Since it only happened with some of the loads, I think that you can rule out the powder.
As well, if they were stored correctly, then that wasn't the problem either.
Based on where the corrosion is externally, you likely had something on your fingers when handling the cases. It looks like a liquid dripped onto the cases. There's no way of telling, but the spots get smaller, suggesting that whatever it was wore off as you touched successive cases or it stopped dripping.
Trace amounts of whatever you touched transferred themselves to the bullets when you were bullet seating later.
You have to stop eating Louisiana hot sauce when you're reloading!
