OL break down, WRs running Bad routes, questionable play calling, coaching, injuries...all are valid problems that contribute to it. I am not excusing romo in that because they are valid reasons.
However Romo also has to take some blame for the performances.
His throws have been high, off target and or eratic. He has a gunslinger attitude when things are going bad and he tries to take too much on his shoulders and make plays when he should just take a sack or throw the ball away.
Now his foot work has declined. I don't know if people remember but coming in from the off season he talked about improving his foot work in the pocket as his big project. How he studied film of Tom Brady in the pocket and how doing the right things with his foot work he would help himself and his oline. Well that all went away from about the Arizona game and when he returned.
Again, bad oline and injuries can cause a QB to be very eractic and make bad decisions, but that can only be blamed for so much. Romo has to understand that even if things are breaking down he still has the option of taking a sack or throwing the ball away instead of throwing it in to double or triple coverage or just throwing it up on a wing and a prayer hoping something good will happen.
He also needs to realize that while scrambling to buy time in the pocket he needs to secure the ball better. This results in most of his fumbling issues. If he secured the ball better while scrambling he would reduce the amount of his fumbles. Now fumbles will happen from time to time even if a ball is secured well, but nothing like what can happen when you do not.
One last thing he needs to work on. He always seems to want to make the big play down the field and then work his way back to the intermediate or dump offs. Now I should not say always as that is a stretch. There are plays designed for the short throws and also hot routes in cases. But more often than not he tries to buy too much time running around while looking for the long bomb instead of taking an obvious first down in a shorter throw.
So again, there are some things outside of his control that have hurt the teams game and his play, but he also has issues he needs to work on to get better.
This is not a blame him for everything or excuse him for everything because it is just not that simple.