To answer your questions:
They don’t give reasons for an interim denial. Like Ed said, even if your record is spotless, you can still be denied one if the agency thinks the position doesn’t warrant it. That being said, I have been in my office and seen 7 new employees hired and receive their interim on average about a month or month and a half after submitting their SF-86. Since I’ve been here, I have been the only one to be denied an interim.
My only issues were some prior MJ use in college and a minor drug related misdemeanor from 5+ years ago.
I had a personal subject interview two months after my investigation begun, and was only asked about the above issues. None of my references were ever contacted.
I cannot receive a CAC until my clearance comes through.