It's probably that Rice isn't yet 100%, & after having something go sproinnnng! when he wasn't even touched, that may be in part mental.

But based on his performance in postseasons past as well as this year, let me offer up this conjecture:
Ray Rice gets in trouble when he tries too hard to make something happen.
One of R2's strengths has always been his balance. I recall one run when the Bengals thought they had him knocked down for a 5-6 yard gain & he pushed up with his free hand & took it another ~30 yards for the TD.

When he's trying to make something out of nothing, he leans too far forward & then when he tries to cut his feet tend to go out from under him. Ball security suffers when he's straining for that extra yard & he loses alertness for blindside hits, so the ball pops out more often.

Is he trying to put the team on his back? Is he worried about job security when he sees Pierce busting big gains? I dunno.

But I think the solution for R2's woes is that he should just stop trying to be Superman & go back to being your everyday Mighty Mouse.