Lots of interesting stuff here, but I suppose the answer to the OP's question is really that nothing is as ingrained as our "natural" swing faults, which have a tendency to reappear when we relax our vigilance. Relaxing our vigilance may be caused by fatigue or distraction, and I suppose one distraction is preoccupation with some other aspect of the game.
So I think I have my drive in good shape; time to really focus on the mid
irons. I therefore think about my mid iron shots a lot, practice them at the range, and when it's time to drive the ball I'm not so focused on that, so my old swing faults reappear. Moreover, it can take a while for us to figure out that this is happening.
In my case, here's a partial list of things that can fall apart very easily without my even realizing it for a while:
1. Posture. I get lazy about trying to keep my spine straight and head up at setup. I revert to a rounded posture.
2. Knee flex. The amount of knee flex I need is more than what feels natural, so unless I make a point of it, I'll stand with my legs too straight.
3. Balance. I tend to put too much weight on the balls of my feet.
4. Smooth backswing. I tend to rush my backswing, making it shorter than it already is.
5. Staying extended/high finish. I have a tendency to shorten my arms, pulling up into a chicken wing.
And that's just a partial list of
my faults that are likely to reappear. Your list might include grip (for some reason, I seem to be very methodical and consistent about grip), stance, or a dozen other things.
The bottom line is, every stroke requires complete attention, and that's just not easy.