Many often refer to over torqueing the barrel, but I don't know anymore about that than they do. Such an explanation may be nothing more than Internet conventional wisdom. If you'll examine the place where they all crack, the alloy is very, very thin - a definite weak spot.

I've had an S&W alloy j-frame crack along with a Colt Agent. What can be said about the very thin place on the S&W also applies to the Colt alloy frame. These older alloy-framed guns really weren't made to shoot much. Two cracked frames convinced me.