It happens that registered users get deactivated for what seems to be no reason. Well that is not the case. the following actions trigger a deactivation:

• Super users de-activate the user in question
• Plugins like admin tools can deactivate the user automatically
• When a users requests his password, the user is deactivated unill he resets it.

The last option makes it appear like this randomly happens, but that is not the case. It is standard Joomla functionalty. Also, clicking an old activation link does NOT act as a toggle, i.e. does not deactivate and activated account. Rather, it correctly returns an "invalid token" error.