Neglecting poor elbow quality, I would say that both KapH and Twar's failure was the result of the diffuser taking a hit to the top on the side and in the direction of the light blue arrow. This would have produce a torque that resulted in a force in the direction of the black arrow at the elbow. This force could have caused a crack to start at the red arrow. This might have happened at any time during assembly, shipping, and/or installation. A very small crack that would be undetectable to begin with could propagate with the vibrations caused by the bubbling of the diffuser until it just gave way.
It's not likely that the cause was from over-tightening UNLESS the threads from the horizontal pipe bottomed out inside the elbow or hit the threads from the vertical nipple of the head. Not likely unless poorly designed from the elbow manufacturer. This is hard to tell without the broken parts in my hands. Can you tell if either threads bottomed out or hit each other?
I recently had a failure on my DIY Vertex base where the PVC nipple broke due to the rope I use to retrieve the diffuser. The rope tied to the upright nipple produced a similar torque below the head. Luckily the nipple broke ($3) and not the diffuser threads.
If the elbow is threaded onto the horizontal pipe, it could be replaced with one form the hardware store I would think.
