I'm aware that this occasionally can happen I'll try and elaborate what happens below, unfortunately I do not have a fix at this time:
When you sign in to comment, a token is stored locally so that we know who you are. Sort of like a session cookie in the world of web pages.
This token is stored in the same place as all your other app settings, the IsolatedStorage for the application. It's a sandboxed area of your onboard memory (or SD card) that the app is allowed access to by the operating system.
Everything that the app caches locally goes to that location, and when the app is finished modifying the location it makes sure to save everything immediately so that nothing should ever be lost.
However, for some people (and it does appear to be less than 5% of our userbase) that IsolatedStorage area is inexplicably wiped out for a reason I do not understand. Either the operating system is clearing it so it's out of my control, or something being written to that area fails in such a catastrophic way that the whole area is lost.
It's incredibly hard for me to test because I can't replicate the problem, it has only happened to me a long time ago on my old Nokia Lumia 800, once. Every device I now have to test with does not have the issue.
It may be that the problem has been solved with the new Windows RT style codebase in Windows Phone 8.1 (not to be confused with Windows Phone Silverlight 8.1) so when we eventually move to that platform hopefully this problem will disappear.