I'm bumping this down. Looks like there is no answer for my question

There are plenty of answers, but I guess Dave isn't present at the moment. Your request for Dave is probably why your question has not been answered.

There are many reasons why BER happens; it's often like a house of cards. The primary (fundamental) reason for this disorder is that the demand of Ca exceeds the ready supply. WHY this occurs can be complicated.

There may be an excess of other cations in the soil, such as magnesium (often applied in epsom salts), or potassium. BER is often induced by heat and drought stress, as well as big fluctuations in soil moisture. Excessive amounts of N in the soil, resulting in a lot of vegetative growth will also invite this disorder. Finally, if there is an inadequate supply of calcium, ready for uptake by the plant, in the soil....there will be a problem. Most calcium products need quite a bit of time for the particles to break down to the elemental level so that they can be take up by the plant.