I grafted some apples onto a crabapple rootstock last spring. These three did fine last year and this spring, but late spring/early summer, the scion shown in the picture abruptly died.

Are there some grafting techniques that are more likely than others to abruptly fail? My current guess is that I had some bark too far down in the graft cleft, and it grew to the point it disconnected the scion from the original tree- but that's just a guess. I plan to add some grafts in the spring, so I'd like to avoid making repeated mistakes.

http://img37.imageshack.us/img37/428/photo3od.jpg

http://img801.imageshack.us/img801/8861/photo4ob.jpg

http://img20.imageshack.us/img20/6665/photo5up.jpg