I have had the same thing happen before on a gun with no mechanical issues. It was the same situation, where I was reloading only one barrel after firing only one shot. The left barrel remained loaded and the left tumbler still cocked when I opened the gun to reload the right barrel. There is much less resistance to opening the gun when only one mainspring is being compressed. In my case, I simply did not open the gun fully because it required less effort to break it open compared to reloading after firing both barrels (in other words, I didn't open the gun with enough conviction because it opened easy). The right sear engaged just enough to hold the tumbler by a thread, but closing the gun caused enough vibration to dislodge it and the barrel fired (in a safe direction, thankfully). I agree you should have the gun checked/cleaned if you haven't had that done recently, but it could simply be a case of operator error as in my case. It brings home two important points: 1)Never close the breech on a gun without pointing the barrels in a safe direction. 2)When opening a fired gun, always make sure you FULLY open the gun to allow both sears to seat properly.
