I think some times when a grafted plant grows a lot it can pinch the graft union and cause that, also sometimes when it grows around the union and pushes down on the stock it lifts the scion and separates the union. Could also be that there was a small infected area in the graft union since it was grafted and that the plant was able to isolate until now but for some reason it got out of control. I really hope you have saved it with the emergency grafting.