You left your security credentials in an unsecured and unattended area. They decided to take this security failure and report it as more seriously than you think is warranted.
If you want to appeal this change, you will need to contact the military.
Personnel Security Clearances Office Information (PSCO) - Personnel Security FAQs
Before contacting them, make sure you will be able to articulate why this claim (which is not a false claim, but depending on the circumstances may be overblown) should not warrant a revocation of your clearance. If a security clearance will greatly affect you, then you may want to talk to an expert in the appeals process before starting your appeal.