I had this happen on one of my Sportsmans Team Challenge rifles. It happened twice in one match. It was a new aftermarket bolt.
Finally narrowed it down to where the underside bolt and breechface come together was a touch sharp. If the round being fed in didnt rise up enough the bolt caught the very top of the case, made it nosedive, and then lit off the primer.
I rolled off the edges on the breechface and cleaned the gunk from the mags so they feed better/smoother/faster.
No problems since.