I have highlighted the stats behind De Gea's decline on other threads, so I won't go over them again, however, I will outline a few theories and general observations below.
De Gea has always been a very unique GK, especially by PL standards. In the PL, you usually want a physically imposing GK. The English game is notoriously physical and there are a number of sides who almost solely rely on crosses, long balls and set-pieces to score goals. Therefore, if you have a GK who can deal with this and take the pressure of the defenders by commanding his area and claiming crosses, that's a massive plus-point.
De Gea lacks these physical attributes, has never commanded his area and is suspect aerially. Likewise, he has a relatively small, relatively light frame for a GK, so rarely throws himself at the feet of attackers when they are through on goal or the ball bobbles loose in the penalty area.
What De Gea has done, and is something that has been deliberately coached into him, is remain firmly planted on his line and rely on his (previously) super-human reflexes to make saves other GKs just can't make. By staying on his line, he gives himself a split-second longer to react to the shot and can turn a weakness (command of area/lack of physical size) into a strength i.e. by saying to the attacker "i'm not going to make your life easier by coming out of my goal, you're going to have to beat me!)
If you want a good example of this, go back to the Liverpool game we won 3-0 at Old Trafford under LvG. That was one of the single-best performances I have ever seen by a GK and I was lucky enough that at the time, my season ticket was East Stand Tier 2, right behind De Gea with a perfect view for most of his saves. That game could easily have been 3-3, Liverpool created a ridiculous amount of chances but De Gea just would not be beaten.
I believe Sterling went through on goal at least 2/3 times and Ballotelli a couple of times as well, and each time De Gea stayed rooted to his line, stood up and then reacted like lighting to get a hand to the ball. There was one, I think from Ballotelli, that he nudged onto the bar which was outrageous. The crowd were absolutely loving it, almost not caring that our defence was a shambles, and actively goading Ballotelli and Sterling to try to beat De Gea...both Sterling and Ballotelli were looking around like "what can I do....this bloke is just a freak!"
The problem is now, as with any player, there's an inevitable physical decline with age. Now, we have just entered the world of Big Data and Analytics, so we know that all the old lazy soundbites about GKs improving with age and peaking in their 30s is just not true. The Data tells you that MOST sportspeople are in their physical prime around 26-28. David de Gea is 30, so should now be well into his physical decline.
Now, this drop off is hard to quantify - I'm not saying he went overnight, but even a 5% decline between 28-29 and a 5% decline between 29-30 would be very noticeable for a GK who relies so heavily on agility, reflexes and hand-eye reaction. Take the Ballotelli save I mentioned earlier, that was a fingertip save from a shot smashed at him from 12 yards, he isn't making that save now, he's probably not even getting his arm up past his shoulder before it's whistled into the net.
So OK...I think most posters can accept that he has never been the bravest, most commanding GK and that there has been some physical decline, but granted, that doesn't explain goals conceded like Barcelona (A), Watford (A) etc...which were just bad mistakes in any GKs book.
My theory is that De Gea now feels a vulnerability he has never felt before, and that is affecting his confidence and his all-round game. Imagine if you've always been very good at your job and you suddenly start to feel vulnerable. You're in a high-pressure environment, the spotlight is on you and you know you're not as good as you once were. You start making little half-mistakes, in De Gea's case you let in the odd goal or two you know you would have saved two years ago. You start over-thinking it, adjusting your technique, changing your body position, you're no longer doing things naturally or instinctively. I think all of that combined with the associated loss of confidence has led to the more serious, comical errors we have seen.
Remember, football is a sport played by millions around the world. De Gea plays GK, a very specialist position. At one time, he had claim to be the best in the world at it. Think about that! Being the best in the world at something millions of people train all of their lives to do! Therefore, my point is, there's a long way to fall! De Gea's decline from world's best to one of the Premier League's worst is more noticeable because of how good he was and how bad he now is - but that drop-off has probably only been 10/15% across the board. Unfortunately, that's enough to really expose a GK.
Finally just to wrap-up, as I said 18-months ago, this is not a blip, this is not a downturn in form. De Gea is just not as good as he was. That's a fact for me. The responsibility now lies with the management to put him out of his misery. It's not his fault he gets picked every week when fundamentally incapable of performing to the standard required. Ole has to make that decision and do what is best for everybody now