/me puts on technical hat
It is odd, the cookies provided seem to have quite long life. I sign in from 3-4 browsers so don't take note of when they require a re-sign in.
It is possible that the hosting provider resets some of the machine in order to keep them snappy which could mean the secret shared between you and the server is lost.
Keeping a user logged in is a nice to have when building system and having the person log in "every now and then" doesn't seem a big hassle.
I have the same issue with gmail every once in a while.
If it is just once a month, don't panic