Why does Clinton Portis always trip

[Rajmahal33]

I have noticed that portis does stumble a lot, BUT he always seems to stumble FORWARD so you can't really complain. It seems like he gets 3-4 yards on the forward fall alone. I feel like he prepares to hit the line hard and biases his body weight forward for this. So when you see him bursting through the line of scrimmage untouched, he is effectively falling forward b/c he doesn't get the anticipated impact. It seems like it takes him two or 3 strides to get his center of gravity under his legs. The alternative could be that he goes to a traditional style of running and when he hits that wall at the line of scrimmage he would be stopped for no gain or maybe even a loss...This would be no good. I guess it's just his style of running but as long as he is producing i see no problem with it.