Why is my tank cracking?
A cracking tank is usually the victim of sudden thermal shock, an accidental drop, or, more rarely, a chemical reaction with certain aggressive e-liquids. If you move from freezing cold to vaping in direct sunlight, the Pyrex can undergo internal stress and crack without warning. Even a minor fall can create an invisible micro-crack that eventually expands with the heat from your coil. Finally, make sure not to screw your tank on like a weightlifter, as excessive pressure on the seals can also weaken the glass.
In short, treat your tank with care and avoid putting it through a temperature rollercoaster. An intact tank is the only barrier between your liquid and your pockets, so a small silicone protection ring could very well save its life.