Most of the users account will get locked from locally desktops and Mobile devices or idle sessions on Server / workstation, We need to start Account lookout troubleshooting from below order.
Perform the below steps on Mobile devices / Smart phone (BYOD)
If user recently changed password and forgot to update in Mobile devices, that cause the account lockout for user ID,
Does user involved has a smartphone or some kind of mobile device using AD credentials for connecting (like exchange), if it fails to connect 3 times (depending on your GPO’s), it locks his account. Have a look on all his stuff using his user account automatically,
specially his mobile (90% of the time guilty).
User below tools to find out source of the account lockout - On Server