What would cause the crack? The first thing I'd do is call McMillan and see what they'll do...then I wouldn't care. Ok, I'd see if the recoil lug was straight and if the action screws were the proper spacing but that's about it.
1. could be a internal fracture or void in the stock. As you shot the rifle, this crack/void continued to propagate.
2. could be that the stock wasn't machined properly and there was uneven stresses at that point...putting increased stress in that area when you shot. Possibly caused from the recoil lug bottoming out on the stock?
How would "I" repair it. I'd get some Marinetex, 2ton epoxy or other tough resin, spread the crack slightly, inject the resin...use a carbon fiber, kevlar, fiberglass cloth over the crack to add strength. In addition, I'd also consider, if I had a drill press, drilling a 2mm hole, properly centered, counter sink it and put a screw/nut to secure the crack. But only if Mcmillan wouldn't help me or if the wait was longer than 3 months.
Just off the top of my head...
Edit...I see that you're in Moscow. I'd just repair it...looking to see if the recoil lug is straight with the action screws and that those screws are properly spaced. Assuming that is correct, I'd simply fill the cracks. I might consider pillar bedding and action bedding as well.