How is this not completely obvious?
If you go back and watch the play, Stew did have two hands on the ball, and then he gets hit and spun around, which caused one arm to break off the ball briefly. And it was immediately after that that the second defender came in and punched it out. It was sort of a perfect storm.
I know because I literally said to my brother while that play was happening, "Look at Stewart cover up that ball, I'm lovin it...OH WHAT THE fug?!?!?!"
I really think the only thing Stewart could have done to avoid that fumble was to get even lower with the intent to just fall forward when the first defender was coming in. Instead, he gets his shoulder low and bounces off the tackle, which ended up leading to the fumble. But man it's tough to expect a runningback to not try to break tackles.
I give Stewart a B on the day. He was the most effective and consistent runningback for sure (though Tolbert barely got any carries). I think DeAngelo is perfect in his reduced role, which still gives him opportunities to break some big, game-changing runs, like the one he had today, which came off a misdirection run. Stewart is definitely better in the base run packages.