AD accounts locking up every minute

Hi there,
I?m having big problems on my domain. user accounts are being locked up, almost every single minute, and I cant find to get a solution for this. I suspect to be a virus, but dont know how to trace it, as I run Symantec endpoint, and nothing has been found.
Hopefully I found on this site the brilliant tool to check and unlock accounts (unlock.exe) which is helping me alot and saving me extra work, but what I really wanted in to understand why this is happening.

rgds,
Geraldo