According to deterioration of explosives, there was some accidents with Tarzon (American Tallboy-based guided bomb) in the Korean War. The bombs exploded even if were safe dropped from low altitude. Deterioration could be a reason. On other hand, US Tallboy had some version and I don't know which one was converted to Tarzon. Early models were build exactly as British, with cast warhead and filled with torpex in TNT mantle, then they switch to cheaper technique with warhead made from rolled plates in process similar to manufacturing high-pressure vessels, steam boilers etc., and tritonal explosive. Breaking the shell when hit the ground could also caused explosion.