There is no OOTB scheduled task that will disable users. I'd double check the user accounts and see if there were invalid login attempts, this will disable a user account. There is also the ability to enable password expiration. Assuming a users password has expired and they haven't changed it, could cause issues logging in and like I stated above, if they enter their password enough times, it will disable their account.
There is also a feature that will delete non-activated users after a given number of days of not being activated (doesn't sound like this pertains to your issue).