I think it's exactly what everyone involved has said, it's a combination of many factors. Linemen missing or failing to hold blocks, running backs improvising and being less than ideal fits, and the loss of Romo and Dez is a factor too, although not an excuse for mistakes. The one excuse that doesn't hold up is injury.
I think the two moves they've made are a step in the right direction. Cassel should help to make the passing game more competent, along with the returns of Bryant and Butler. And Collins should be an upgrade over Ron Leary.
And I think they need to see what they gave or dont have in Michael too. Randle and McFadden have had their shot with inconsistent and disappointing results.
One thing I would also be doing that they're apparently not is looking for upgrades over Street and Whitehead. I don't feel either is capable of giving this current team the help at the position that they desperately need.
That's my thinking, what about you?