I'm thinking 3 piss poor managers in succession really left De Gea and a few players in the squad with a feeling of 'why bother if no one else is?'. Players begin to get cosy. Happy to pick up the salary without willing to sweat blood.
The likes of Pogba, Lingard, De Gea, Martial, Lukaku entered into an unhealthy comfort zone under that is very hard to snap out of.
De Gea got his ridiculous final contract based on being brilliant for a a few seasons even though a season before he signed the new deal he was clearly not focused like he once was. That was put down to being distracted by lack of progress in his contract talks, and it was believed he'd be back to normal once he signed on the dotted line as the world's best played keeper by far.
Anyways he's absolutely mugged the club, his motivation is not what it was now that the ink has dried on his final Man Utd mega contract. We see it time and time again with pampered stars, once the ink dries on that final contract, then they remain in their unhealthy comfort zone, happy to enjoy their life more than before, only give 80% of effort as the millions roll in. Ozil, Alexis Sanchez, De Gea all examples