All of these forms of crack development are the result of the cylinder being subject to high pressure for prolonged periods. The cracks are intergranular and occur at grain boundaries. There is no evidence of stress corrosion or fatigue.
The presence of a relatively high lead content has been identified as a contributory factor. Cracking at the grain boundaries is accelerated in the presence of lead. The presence of bismuth is also suspected to be contributory.
Alloy composition has also been found to be a factor. Alloy 6061 has shown good resistance to SLC, as have alloys 5283 and 7060. 
Manufacturing defects such as folds on the inside surface have been shown to be harmful, particularly for parallel-threaded cylinders.
Grain size has been shown to be of relatively minor importance.