The most common causes include, but are not limited to:
Going in and out of games too quickly (most common cause)
Going in and out of accounts/characters too quickly
Spamming text in the chat channel
Overuse of the mouse wheel
Repeated use of an incorrect password may lock an account login for your protection
Repeated attempts to use an invalid CD-Key
Rapidly connecting/disconnecting to Battle.net
Repeated use of third-party programs (hacks) - these include but are not limited to sending bad packets, flooding the server(s), and the use of hack programs
Temporary restrictions will last anywhere between a few minutes and 72 hours, and repeated logon attempts may result in a longer restriction.