Iusr account getting locked out

IUSR_Servername keeps getting lock out any ideas?

Logon Failure:
Reason: Unknown user name or bad password
User Name: IUSR_Server
Domain: COmpany Domain
Logon Type: 8
Logon Process: Advapi
Authentication Package: Negotiate
Workstation Name: SERVER
Caller User Name: NETWORK SERVICE
Caller Domain: NT AUTHORITY
Caller Logon ID: (0x0,0x3E4)
Caller Process ID: 3156
Transited Services: -
Source Network Address: -
Source Port: -


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