I'm not familiar with the E53 and the fan/radiator clearances, but typically on other vehicles it is due to loose bolts connecting the fan or a failed viscous coupler which allows the fan to have too much freeplay forwards and backwards.

How are the belts? On Occasion, a broken fanbelt can also get entangled in the fan blades, breaking them.

And sometimes a fan blade just cracks and fails, causing failure of other fan blades.