I suspect those few clean patches didn't get enough of the oil out before you loaded it. They will seldom get the oil that resides at the front of the plug right against the barrel. It would take very very little oil to contaminate some of the powder over a ten month period.

When I'm loading on a clean barrel and expect it to stay loaded for a while if I don't get a shot, I swab with a very wet alcohol patch and then a dry patch or two. Then after I load the bullet I run a lightly oiled patch down to the nose of the bullet, then a dry patch. Never any rust and always a full charge BOOM.