I think it's poor habits that have become ingrained in his game through the years. He did not use to play with this lack of urgency and laziness/cheating so much when he first came in in the first couple of years. He was actually a decently complete player with better and more consistent effort level and battle. But over the years he got checked tighter and to continue to put up huge points he's "learned" some bad habits of cheating and laziness (to preserve energy).... fly bys to get a jump and or not have to stop and start to save energy, save energy with poor effort in battles or just lets others battle for the puck while he shuffles off and cheats to open ice, not the same energy level on defence again to save energy for offence.
Just bad habits ingrained with those super long shifts he and drai used to take. You cannot play that many 1 plus minute shifts in a game and not coast, cheat and lack effort in defensive responsibility. Unfortunately these habits can be hard to shake off and it becomes your default game.
I felt like it kind of became quite noticeable sometime during the 17-18 year (i noticed it a bit in the 16-17 season especially the playoffs but it wasn't extremely obvious) but was very noticeable in the 18-19 year. It's kind of backed up by the huge drop in plus minus even though that's his high mark in points in 18-19.
I remember McDavid being pretty decent defensively when he came into the league and its just eroded from there. I remember him challenging for pucks and using his stick/good hands to win a decent amount of 50/50 pucks.
Two years ago (I think), he really started (ridiculously) cheating for offense. He did a lot of fly-bys. To be fair, we didnt have much depth and if McDavid wasnt scoring, we didnt have much chance. He was in a bit of tough position.
This year, he has been pretty awful defensively. He pretty much hasnt engaged defenders all year. I personally think it was because of his knee injury. I dont think he wanted to get hurt. I think its fairly understandable if thats the case.
We have much more depth this year than in the past, I think McDavid needs to try to adapt his game a bit better to use his linemates and play more of a 200 ft game. If hes healthy now, I think he needs to focus more on the details of the defensive game and have more impact all over the ice rather than just in the offensive zone.
Hes an amazing player but I agree I think he has picked up some bad habits over the last few years. When hes getting caved 5 on 5 ... its not a good recipe for success.