Looking at the pictures of the break, that looks like a 24" bucket, and there is much wear on the rear lip of the bucket, with the paint worn to bare, shiny metal. That kind of suggests a lot of prying work, such as using one side of the bucket to break or cut roots (Speculation on my part, but I know that happens a lot when I do stump grubbing with my BH). When you pry like that, the resistance to the curl and/or crowd forces of the BH is offset to the side of the bucket, which applies a twisting force to the boom assembly. The wider the bucket, the greater the twisting force. It may be that a 24" bucket is too wide to be used extensively for that type of work on a 9', CUT BH, and maybe 2 years of heavy use like that is about the limit. Can't say for certain, but we've used a smaller 18" bucket for similar work for more than 10 years with no such problems.
Not trying to be critical of the OP here... just offering a suggestion for why this might have happened and a possible solution.