The ease of fouling plugs is directly proportional to how desparately you need the car to start.
Lesson learned! The hard way
You had the perfect situation for fouling plugs.
Rich mixture, short run time, some oil in combustion.
If the porcelain of the plugs does not get hot enough, when you shut the engine off and they cool the wet gas/oil that didn't get burned off crystallizes when they cool. They won't necessarily quit while running, but will fail to re-start after cooling.
Does that sound like what happened?
That's what happened to me right now. Messing with the charging system and had it run shortly for a couple of times. She won't re-start now. Pulled the plug and yes, wet from fuel/oil.
See if I can find plugs tomorrow.