I have the same issue. Sometimes I may lay it down on the bed to read something while snacking on something, and 95% of the time it changes to landscape mode. Bugs the heck out of me so I try to use a pillow or something to prop it against instead. Honestly, I think it's a sensor that's overly sensitive on one side or the other, because it takes forever for it to flip back.
Something I've delt with from day one and probably will until the next nexus comes out in a few months.
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