why user logged out from activedirectory automatically?

I am using the data in active directory to authenticate user. However, some users are usually automatically logged out by no reason.

Anybody knows why?

My code is simple:

LdapAuthentication myAuthentication = new LdapAuthentication(path);
bool Authenticated = myAuthentication.IsAuthenticated("myCompany", username, password);
                  
if (Authenticated == true)

{

 .....

}
...