Some 3 point wind convergent going on there David, where moist air congregate to feed the mid level moisture layer, though a dry layer in-between is evaporating it before it makes a visible low feeding updraft. may have become more intense as time went on tho moved away from line of sight by then, or it may have just collapsed ?
Very keool formation tho, Storm chasers would be interested in it also as it is quite unique indeed
Great Catch