Its a bug...your phone's home screen is supposed to only default to landscape if you have media mode (dnla) activates. Mine does it occasionally, but only when I have it lying face up under certain types of light (mostly at work under the fluorescents). There's no good way to keep it from switching or get it to change back....one of the (not so many) reasons I'm considering swapping from a galaxy s 3 this week....well see...if HTC acknowledges these issues soon...I may stick...but the whole "mums the word" on this and items like the wifi issue are starting to add up
Sent from my Evo LTE using 3g...because Sprint needs to get their collective asses in gear