Why does magnetic charger fail

Anything from a random failure in the electronics embedded in the charging puck and USB plug, through to someone tripping or pulling on the cable and breaking the physical connection into one of the ends.

Charging puck is mainly a coil of fine wire which can break at any point.

Pretty much everything nowadays contains electronics and microprocessors, including that charger, and electronics and microprocessors can all fail.

For an intro to some of the potential causes of electronics failures, start here:

… https://en.wikipedia.org/wiki/Failure_of_electronic_components