A number of different factors.
Firstly, a lot of teams have tried to shoot from pretty much anywhere on the pitch. This also skews the statistics regarding the shots on our goals. Often, the media highlight that number as a result of our poor defence rather than the fact that shots from 25 yards aren't actually chances, often aren't on target and highlight a good defence yet all are combined as "shots on goal". I'm not saying that we haven't leaked far, far too many chances, but sometimes statistics don't tell the full picture. A shot from 40 yards that dribbles into the keeper is a shot on goal, the same as a one on one that the keeper saves, now tell me, which one is the actual scoring chance?
Secondly, it highlights more of a problem with the space often left by the midfield rather than the poor defence. I can't recall many one on one saves that David has had to make. Again, this isn't a definative reason why I believe the reason for the opposition having so many "chances" is more down to our midfield than defence but it doesn't help that we normally leave acres of space for the opposition to exploit. Which often leaves too much for the defence to deal with and has often needed De Gea.
I said this earlier in the season that we will concede more chances and goals just because of the way the midfield has played in the first few games. The game against City, I think 3-1 would have been a fair result. The last 3 goals we conceded I think overplayed City's "dominance" over us. Not saying they didn't deserve the win but it wasn't a true reflection.