exhaust manifold stud just broke

Have a 1974 Atomic 4 gas engine in my sailboat.
One of the exhaust manifold studs PULLED OUT IN MY HAND.
I'm not that strong (!) and have not used a tool on it ( that I recall :))
It was broken at the threads. Does this "just happen " ?

Yeah, it happens...I've seen it happen probably.....5 maybe 6 times. Every time I saw this happen it was a broken bolt where-what would be the hottest part of the engine area.