From my bartending days i can tell you i have seen it happen many many times with bottles from A-B, Miller, Coors etc.. I usually attributed it to the opener catching the that glass lip instead of the cap and breaking the bottle.
I have seen some flawed bottles on the bottling line that have shattered when the caper came down on them. It is usually easy to pick out the flawed bottles with a visual inspection, the flaws look like little clear spots, basically an air bubble in the glass. If you find flawed bottles, you need to switch bottle suppliers. You may ask the customer what kind of opener he used, if its one of the bartender style openers that may be your cause. The opening on those is large enough to catch that lip.