My understanding of this form of canker (there are other types) is that it is bacterial, called Pseudomonas syringae spp. The bacteria infect lesions in the bark caused by insect attack, then feed on the timber spreading inward to the pith. As they spread inward, developing their own cell growth, they grow a cork-like material (it isn't bark, at all) which is substantially weaker than the host timber. If the lesions exist in any number, particularly if they encircle a branch, then that branch will inevitably be so weakened that it will fall. Lesser infestation can result in a cankerous growth lasting for decades within a branch.
Les
