It was both of them, just look at the four game stretch on October. Even though the OL played horridly, regardless who was playing RB, they were getting no help even in max protect. At the time Williams was getting no burn at all.
Lets face it, we have three backs with strong splinter skills, but none are a complete back (and I'm not even talking about elite). It also limits the substitution patterns, whoever is in the game, telegraphs what we're capable of doing.