Joe,
Although it's possible a remaining ember set off the charge, I'm a little dubious. This has happened at reenactments, but it happens immediately when the new charge is being poured down the barrel - not when the musket has set for awhile. And of course, a lot depends on the powder. CS powder from the Augusta Works was said to be as good as commercial British powder, which was of a higher quality than most black powder available today. But who knows where your ancestor's cartridges came from.
Another possibility - and I have seen this happen myself at a reenactment (thankfully, no injuries, other than possible soiled trousers). The hammer could have been at half-cock ... by the manuals, it was supposed to be at half-cock after loading (the only "safety" on a musket). If the sear spring were weak, or the nose of the sear worn down, or the half-cock notch in the tumbler worn out, or more likely a combination of these faults, some sort of jarring or movement could take the lock out of half-cock and possibly fire the musket.
In the instance I saw, some moron was just idly tapping his loaded musket (blank) by the butt on the ground (obviously not even thinking about what he was doing), and the musket fired into the air. I don't know what the exact failure was (except common sense), but I'm sure his lock had one or more of the above problems.
Geoff Walden