How come the wow account i use get locked

There are several reasons why an account can be locked. Note that this is different than the account being banned. Usually an account is locked not because of breach of terms or behavior, but as a security measure.

The system retains information on your location when you log in; if you suddenly log in from a completely different country, the system locks your account as a precaution, to make sure it isn't compromised and being used by illegal parties.

It also does this if your account performs other "suspect" activities indicating it might be hacked or compromised, such as being logged in simultaneously from two locations.

Usually, going into your battle.net account and changing your passwords helps to undo the lock. If this doesn't help, contact Customer Support to have them sort out the issue for you.