The round definitely hadn't started to chamber, it was completely to the rear of the magazine.
It is possible that my thumbs forward grip led to the issue. I've never had this issue with any standard pressure ammo, so I'm willing to believe the more powerful ammo combined with my somewhat faster shooting could have caused my thumb to move to the slide release.
I will get back out and fire the rest of that box and see what happens ASAP.
Know the status of your weapon
Keep your muzzle oriented so that no one will be hurt if the firearm discharges
Keep your finger off the trigger until you have an adequate sight picture
Maintain situational awareness