It would seem to result from the pursuit (by all manufacturers, including Apple) of thinner and thinner phones, which means you either (like Apple) compromise battery life, or you (like Samsung) try to increase battery efficiency to the point where stability is an issue.
The daft thing is, I know loads of people who complain about short battery life, but very few who wish their phone was a millimetre thinner. As witness all those add-on battery packs, and cases.