clarithromycin (and other antibiotics) is a powerful inhibitor of enzyme CYP3A4
the increased sedation is likely due to this inhibition which causes higher blood concentration and increased duration of diazepam
it's generally safe combo as long as you are tolerant to that much diazepam, but you should expect increased sedation and amnesia... so don't drive anywhere and stay home
I don't believe they interact any other way, but I am not expert