Hmm, offset spokes, and the crack didn't involved the opposite-side spoke hole right in the middle of the tear. Well, it must have been fatigued. A neighbor noticed longitudinal cracks in a rim he'd been tightening, and probably over-tightening, for years. If it had let go it might have looked just like that, but that's just a guess. FWIW, Jobst Brandt's book says that the rim is the weakest part of a well-built wheel and this it will eventually fatigue. This wheel looks to have given all it had to give, lived a long life.