If a modern revolver is fully cocked, or if its hammer is down due to a trigger pull, its cylinder should remain locked in place and should not be able to rotate.
The possible causes of improper rotation might be:
• Bad timing, in that the cylinder locking bolt rises in the wrong place, or at the wrong time, to allow it to rise into and secure the appropriate cylinder notch,
• Weak locking-bolt spring, which allows the bolt to be displaced out of its notch by movement of the cylinder of any kind,
• Oversize (or even undersize) locking bolt, which keeps the bolt from securely remaining within its appropriate notch, or
• A locking bolt with rounded edges (that are supposed to be sharp), allowing it to be forced out of the notch by cylinder rotation.
In any case, the revolver is defective and at least potentially unsafe. If it is within warranty, Taurus should repair it at no charge. Contact them for instructions.
If it is out of warranty, a good pistolsmith can repair it, but the job may be expensive.
What say you, Eli: Am I on the right track? I believe that you know more about revolvers than I do, so please chime in.