Apparently, a lot of people are having this issue.
1) Look in your HB2 installation folder, and you should find a file called
StyxSettings.xml.
2) Bring the file up in Notepad
3) Locate the following lines, and change them to something that works for you:
<!--Whether or not we should log out after the bot has detected inactivity.-->
<LogoutForInactivity>True</LogoutForInactivity>
<!--Logs out after X minutes of inactivity.-->
<LogoutInactivityTimer>10</LogoutInactivityTimer>
By the way, this information was in the release notes for
HB-1.9.5.7:
ADDITION - InactivityDetector added, can be set to log you out and/or quit wow if stuck for too long. ( configure it in * StyxSettings.xml )
cheers,
CJ