Bet it felt good to take that doe with something you made yourself though!
I'm pretty puzzled by a cast breaking up in a deer like that - supposedly one of the benefits of cast is staying together.
I've never cast my own - mostly because I have worked around plastics and I know all teh little crap you have to master before you can put out a quality product. If I might hazard a guess - it sounds like you may have had a void in that bullet. My reasoning (which may be flawed) is that mold is rated to crank out a 255gn bullet - your stated bullet weight was 10 grains shy of that. How do your other bullets compare?