I'm no expert.... but this may be what they call Splash Fouling?
Below from some good web sites
Mark
Splashed fouling, Small islands of contaminants on the insulator indicate splash deposits. Replace with new plugs of the correct heat range. The use of fuel additives, carburetor and choke cleaners or other aggressive solvents before installing new plugs is the most common cause of this condition. This may sometimes occur after a long-delayed tune-up. Here, deposits accumulated after long periods of misfiring or low power operation may be suddenly loosened when normal combustion temperatures are restored after a set of new plugs has been installed. During a high-speed run, these materials shedding off the piston are thrown against the hot insulator surfaces
427 S/C, Flame Yellow, B2 Motorsports 427ci Stroker Dart SBF, TKO600, 3.73 Posi, Avon tires and so much more.