Ground causing / not causing a fumble.

[MCJB Ump]

A couple of plays in the Steelers - Ravens game got me thinking. Why can the gound cause a fumble if an offensive player loses control of the ball while hitting the gound without being touched but if a QB goes into a slide and loses control it's not a fumble?

The Steelers Willie Ried went down on his own after losing his footing on a KO return and it was originally ruled a fumble. Replay overturned it bcause he was touched on the back a split second before he lost control. A little later Ravens QB Troy Smith went into a slide and clearly fumbled while going into the slide but was called "down by contact" without ever being touched. I can understand protecting the QB but I feel that should still be a fumble.