Another "theoretical" possibility with excessively wide tires on narrow rims is that tensile stress can predispose the rim to failure. I've had this happen on 3 consecutive 406-16 rims (Kinetix) on my folding commuter bike. Tires were 406-47, and this only happened on the rear wheel. The first sign of trouble was a "thumping" sensation on application of the rear brake. I initially thought this problem started with brake track wear that propagated into the center web of the rim, but one rim that failed this way had minimal brake track wear. I showed the failed rims to Bill Mould and he advised a wider rim. I took this advice and the current wheel has passed the mileage the others failed at, and appears to be trouble-free.

Tangential view

Radial view