The last three midnights I've gotten an Event ID 539 in the log...about my own account:

Event Type: Failure Audit
Event Source:   Security
Event Category: Logon/Logoff 
Event ID:   539
Date:       2010-04-26
Time:       12:00:20 AM
User:       NT AUTHORITY\SYSTEM
Computer:   SERVERNAME
Description:
Logon Failure:
    Reason:     Account locked out
    User Name:  MyUser
    Domain: MYDOMAIN
    Logon Type: 3
    Logon Process:  NtLmSsp 
    Authentication Package: NTLM
    Workstation Name:   SERVERNAME
    Caller User Name:   -
    Caller Domain:  -
    Caller Logon ID:    -
    Caller Process ID: -
    Transited Services: -
    Source Network Address: -
    Source Port:    -

It's always within a half minute of midnight. There are no login attempts before it. Right after it (in the same second) there's a success audit entry:

Logon attempt using explicit credentials:
 Logged on user:
    User Name:  SERVERNAME$
    Domain:     MYDOMAIN
    Logon ID:       (0x0,0x3E7)
    Logon GUID: -
 User whose credentials were used:
    Target User Name:   MyUser
    Target Domain:  MYDOMAIN
    Target Logon GUID: -

 Target Server Name:    servername.mydomain.lan
 Target Server Info:    servername.mydomain.lan
 Caller Process ID: 2724
 Source Network Address:    -
 Source Port:   -

The process ID was the same on all three of them, so I looked it up, and right now at least it maps to TCP/IP Services (Microsoft).

I don't believe I changed any policies or anything on Friday. How should I interpret this?