Since this happened while you were shooting it, I'll assume you didn't tighten any screws.
It sounds to me like the trigger plate the set triggers are built into has moved upward a bit. This could be caused by over tightening the screws that hold it in place (which you didn't do) or a combination of the wood shrinking over time and the shock of the recoil jarring the plate just enough to cause trouble.
I suggest removing the trigger guard and trigger assembly and then cutting a piece or two of heavy card stock, the kind that cracker boxes are made from, so that it fits down into the trigger guard mortise between the trigger plate and the wood at the rear of the trigger plate.
Putting these card-stock pieces in place and reinstalling the trigger should fix it. Check it out before you reinstall the trigger guard. You might have to put in more than two pieces.
Some set triggers also have a screw on the spring at the rear that can adjust the spring tension. Changing the tension might also fix the problem but be careful. If you reduce the spring tension too far, the rear set trigger won't be able to trip the lock sear to fire the gun.