Accumulation as described by Carl, enhanced by infrequent oil changes of non-detergent oil.
And, as I think Carl alluded-to, not running your engine for at least 30 minutes after the engine warms up (in other words, short trips).
I have been accused of many things and have been called many unsavory names, but an alluder?! That will not stand!
For shame, Tim! I thought we were good.
Carl