Some of my users get their accounts locked out while in the middle of working. I don't suspect any external hacking but I can't figure out why they are getting hacked.

I'm getting the same thing, except even though I adjust the user properties with user manager... they still show as locked out on the users end! One particular user reported being locked out but when I reveiwed user manager, it showed they were fine.

Have a look in the event log of your PDC. This will tell you on which workstation the lockout event occurred. Is this the same machine that they are using when the account lockout is reported. My guess is not!

I would imagine they are still logged in at another machine somewhere and while they have changed their password elsewhere, this PC keeps locking their account.

Also check the user's login script on one of the Domain Controllers as well as all drives that are currently being mapped when they log in. If a user's NT permissions or group membership has changed then the rejected mapping of drives will lock the user account.
The same is true for printers, as they are also a shared resource.
Disconnect all the users currently mapped drives and uninstall all network printers from their profile. Have them log out and back in again so they can rerun their login script.