Periodically, I see articles which report catastrophic failures while shooting. An example is a recent article at The Firearms Blog concerning a P320 which blew apart (but, was, with some minor repairs, still used to complete a competition). In the majority of situations, including this instance, the failure is blamed on a "double load" of powder in a reloaded cartridge.
I suppose it might be possible to "double load" some cartridges, but the casing of a 9 mm is so small compared to its powder load that it is impossible to have a true double load. You would have powder spilling all over. There are several things that can happen, though, to result in case failure.
One cause, which I've seen it myself with my own reloads, is higher pressure resulting from a change in powder; not the type or brand of powder, but a different manufacturing lot of the same powder. It is not unusual to browse through reloading manuals, and see the maximum recommended load for the same powder and bullet combination decrease over time as the manufacturer tweeks their formulas. Don't expect a recipe from a couple decades ago to still be valid for newly purchased powder. It may be, but it is not a guarantee.
Another cause, particularly when reloading spent brass, is a weakened casing: a crack or other defect that suddenly gives way. This requires careful inspection of the case at all points of reloading. I have, for instance, discovered defects even up to the point of measuring powder into the casing.
And I've heard from those using electronic powder measures, of irregular amounts being metered. For instance, my father-in-law recently had to replace an RCBS combination powder measure and scale because it suddenly started measuring out erratic amounts of powder.