Looking at how they are mounted, it doesn't seem that unusual to me
(who has almost zero hours as a B-52 engine mechanic) that a single
engine might decide to defect to North Dakota.
Might have been improper mounting to the pylon, or failure of same. If
it was an uncontained failure of the turbine, I'm guessing the crew
may well have shut down the other engine in the pod in case of
collateral owies.