I don't know the answer, but I think it's happening to me too. More often than not I am having to sign in now, even though I keep ticking the box to remember the sign in.
Have you changed your computer operating system? I assumed in my case it was because I'd just recently got a new computer with Windows 7 instead of Windows XP, and an updated Internet Explorer, so I wondered if the problem was something to do with a change of operating system and tightened security measures, rather than any problem with HubPages?
Sorry I can't answer the question, but I'd also be interested to know why the sign in isn't being remembered every time, as it used to be.