It's a natural tendency of primers to want to come out of the case, the pressure that's pushing the shot down the barrel is also pushing back on the primer. In a typical situation the pressure is also pushing back on the hull, too. The hull and the primer are both restrained by the breech face, so the primers typically stay put.
The first time I experienced primer back-out was with an old .45 revolver that I was using as a cap gun. I experienced cylinder drag with primers only but not with full loads, so it got me to thinking. It seems, with only the primer, there wasn't any pressure on the case -- the primers were pushed back to the recoil shield but the cases weren't. There was enough slack to allow the primers to back out a little and cause drag. If the hull and primer aren't uniformly restrained by the bolt face, the primers may back out.
Since you've eliminated ammo problems, for your 11-87 to have primers back out two possible causes come to mind:
The face of the bolt isn't uniform. A dimple around the firing pin hole could allow primers to back out. But, more than likely, the bolt is opening prematurely while there is still sufficient pressure to loose the primers. Primers backing out is a tell tale sign of wear at the locking block and the corresponding barrel recess. Remington has an over-sized L-marked locking lug that may solve your problem. In some cases you may have to replace the barrel, too.