I've seen a number of posts on this, and it seems that there is no clear reason. I regret that I am about to add to that confusion.
I have a 2009 Jetta TDI with 140k miles. Love the car.
For about a year now, I have been getting a warning messages about once a week, "Engine Fault Workshop".. Sometimes, the grow plug "spring" would also light.
After a few minutes, the error would go away. This did not appear to effect performance of the car in anyway (That I could tell).
On a couple of occasions, the car logged a softcode suggesting a problem with the turbo charger position sensor. This was replaced as a result.
A few months ago, the right head light of the car stopped working (both upper and lower and turn signals). After two hours at the dealer, it was discovered the fuse blocked needed to be replaced.
Seems the fuseblock had corrosion on it that caused at least one of the fuses to melt the plastic of the fuse block (the corrosion increased resistance but not amperage so the fuse did not blow).
After replacing the fuse block, to my surprise, I have not had this error occur again.
So, if you found this from a Google search (or similar), take a look at your fuse block and make sure everything is making a good connection. In my case, it would seem an intermittent loss of power to the engine systems may have been causing my issue.