Sounds like the locking bolts on the bottom of the barrel are worn, dirty, or the spring is weak. I would start by cleaning the locking bolts (or lugs as some folks call them) and the channel that they slide back and forth in. You can drive out the pin that holds them in, be sure not to let them pop out and fly across the room.
If your barrel is old enough to have the 1 piece locking bolts, you might want to replace it with the newer 2-piece split lugs. Some of the older barrels also had a set screw to hold the bolts in rather than a roll pin.
If the bolts are clean enough, but still having the problem, then I'd bet that the spring is weak. This part should be available directly from TC, usually for free.
With the way the gun is designed, this is not supposed to happen. Theoretically, if the locking bolts do not engage the frame enough to keep the barrel closed when fired, the hammer is not supposed to cock. However anything can wear, and the interlock inside the frame is fairly precise, subject to wear after years of use.