It's certainly a mediocre mount design at best, but it would be arm chair Quarterbacking at this point to try and give a solid answer why it failed. We've never seen it before, but we prey for a speedy recovery to PQ!
Long story short we're not exactly sure, it's definitely is a strange issue. The rear control arms are cheap, only about 35-50 bucks. I'm thinking it would be just as expensive to just replace the arm as it would be to hire someone to weld the damaged mount. We would replace them both for a little more piece of mind.
Happy Holidays from the Pfadt
Family to yours!