I'm reasonable, lol?
Muscle strain or tear on a very fast player could happen at any time, more so earlier in the season. Some players will have it their entire careers. DC is what, 27, 28, so he's coming into the right age for a player like him to suffer more with it.
Other injuries can be caused by any number of things. Repeat muscle strains for lots of players probably is down to something in training or conditioning. Also could be because we generally have one of the oldest squads. It's not really a surprise to see it happening more for players like Chiellini or Khedira.