I have seen a number of threads over on the Mobile App section of the Community (including the one below), and it would seem there are a lot of folks that are irritated by this Mobile App behavior. The App would randomly log users out. In my case I rely on critical notifications being texted to my (Android) phone, so if the App logs out then those notifications are lost!
Since I have not seen any Staff response to those posts, let's make it official:
Please consider this a request to fix this log out bug, or alternatively consider it a feature request to stop that behavior.