I posted a while ago about this noting I saw it happen quickly once or twice and was wondering what was up. HOWEVER, I subsequently bought the multimedia and car docks which accept the phone sideways and clearly change the orientation to landscape and apparently use a magnet in the dock to do so as the phone rotates the screen as soon as it's an inch or two away from either dock!
I believe the times I saw this prior to using the docks was because the phone was in proximity to a strong magnetic source that tricked the phone into believing it was in range of a dock and it momentarily rotated.
So my guess is this is what's happening. Remove the magnet or whatever is causing it to think it's in the proximity of a dock and the home screen will change back to portrait.
Don't believe me, find a dock and move your phone close to it and watch it change from a portrait home screen to a landscape one!
I believe the times I saw this prior to using the docks was because the phone was in proximity to a strong magnetic source that tricked the phone into believing it was in range of a dock and it momentarily rotated.
So my guess is this is what's happening. Remove the magnet or whatever is causing it to think it's in the proximity of a dock and the home screen will change back to portrait.
Don't believe me, find a dock and move your phone close to it and watch it change from a portrait home screen to a landscape one!