cam shaft

Have a 1995 Lumina 3.1 that has 115k on it. Well maintained vehicle. The cam shaft broke just past the third lobe. Any one have thoughts on why this might have happened. Lobes and bearings look good , not burnt like lack of oil. Has anyone heard of cams breaking lately like in the old days. I think it was a poor casting. Chevy was no help and of coarse no support for the age and milage. Was not overheating and no antifreeze in the oil. Car quit while on highway. Thanks for feed back.