Looking further into inert cannon balls exploding, it would appear this is based around chemical rectors within the iron and the salts from being submerged for many years. This can cause them to super heat and fracturing, which on a large ball such as this can make quite a bang. However, trapped chemical gases, formed under the high pressure of a deep sea environment, within the iron casting, can form micro pockets that when brought to the surface can be under extreme presume due to the loss of sea depth compression. These are the ones you have to look out for as they will explode. This can cause a chain reaction throughout the casting, causing the ball to explode with an effect similar that that initially intended as an artillery projectile. The fact that this had lain on the beach for at least a fortnight and in fairly shallow water means the chance of it exploding would be fairly slim?
I'm quite interested in the fact that a ball this big appears not to have been a favoured poundage (128lb) of the British, but we're largely used by American coastal defensive pieces known as a Columbaird. If so, what was it doing on an East Kent beach? I'm aware this could have been from a sunken wreck, but does anyone know if we, the British, we're using land based pieces of this size?