SQL Server Failed Login - SQL Server login (not windows) - Random times
I would look for some kind of automated process... either a SQL Agent job, a batch file connecting to SQL, etc.
There is some good chance that the login use for this process does not have the right password. 

You can create an extended event (or a trace) and capture the "Error Reported". Make sure you include the "application name" in the collected field and you should see something like : 



This should give you some good info to find the cause of your issue.