Cause of slam-fire in my 6.5CM AR10?

[mel]

this was actually in my ar-10 cm handloads with a cci #200 primer , first time loading the round inserted the mag , had the gun down on the back of the flatbed on my bipod getting ready to shoot some 5 round groups , just dropped the bolt with the release like normal and it fired, while my cm and grendel are both fairly new to me my 5.56 ar has a few thousand rounds threw and ive never had it happen with it i load for that with cci 400's