If its vCSA 5.5, it is likely to be this,
VMware KB: vCenter Server Appliance 5.5 root account locked out after password expiration
For other versions there are few possibilities I can think of
Do you have any password management/compliance utility , these tools have a tendency to reset password regularly
Or
Have you configured password aging
Interesting thing to note is that on a reboot the password is recovered, strange...