A number of factors can cause your account to logout. These include network roaming, inactivity and other factors controlled by your Internet connection, such as proxy settings.
Passwords are automatically reset whenever a fourth new IP range is used to access your account. This is done to deter account exploitation.
If you work from multiple locations or computers, check the "Keep me logged in" button (and don't click logout) and the website will keep you automatically logged in and bypass the IP check. You might need to log off and sign back in to see this option.