I have a few plugins running on my desktop -- recaller, bluewswich, calendar, personal data plan monitor.
Once I unlock device and saw all plugins have gone away.
I took a look on /home/user/.config/hildon-desktop/home.plugins.
It was empty.

I wonder what background process and WHY accessed this file and erased it?

How do know what happened?

The cause, now that would be the million dollar question. There are many theories, for every widget that exists there is one person saying it is buggy, another saying that it is not.