Have you ever edited your options/preferences from the mobile/smartphone version of the website? The mobile skin version of the preferences page is missing the "Miscellaneous options" box that is present on the full desktop version of the same page, and apparently related to that when you save options from the mobile version it resets your editor to the Basic Editor (although it seems to preserve some of the other settings, which is odd)

There may be other mechanisms by which this occurs, but the above is definitely one of them.