It happens when you sign players in their thirties and give them a massive contract. I wouldn't sign players aged over 25 for big money(avoid buying them at all price, with some possible exceptions).
Varane was a classic Woodward signing, Casemiro wasn't(because Woodward wasn't at the club anymore) but it feels exactly like it. Arsenal buys Declan Rice, Chelsea buys Caicedo(and Kante before that), City buys Rodri and we buy past his prime Casemiro who's in his thirties and physically spent.
Manchester United just suffers the consequences of it's stupid actions. Nothing less, nothing more.