I had a set of those break in 10 street miles. The company that sold the axles quickly replaced them claiming poor heat treat. The replacements never were a problem but my thinking is the design is poor at best. All axles I order I ask for a press in stud. My studs were not rusty like yours and I always use lube on the threads but have been told not to do so. I'd guess(only that) your lug nut seized on install and when you hit it to remove it the stud broke.