AD account lock out event id 4711

Network Information:
    Client Address:        ::ffff:192.168.3.40
    Client Port:        50853

Additional Information:
    Ticket Options:        0x40810010
    Failure Code:        0x12
    Pre-Authentication Type:    0

Its hard to say why the account lock out automatically in AD.Best way to go is to reveal the real client IP as described above and examine that machine what is running on it.

Download the account lock out tools from below link and monitor the reason to sort it out.

http://www.microsoft.com/en-us/download/details.aspx?id=18465