Apparently it's because the original architects didn't factor in the weight of all the tourists who visit it.
Well, no. Not really.
According to the BBC, the real reason is that, "The building's foundations require a steady stream of moisture from the Yamuna River to retain its strength - but the river is slowly drying up." But the headline immediately reminded me of the urban legend of the sinking library.
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I'd expect the ground settling due to water receding and leaving voids behind to be a problem, too, but maybe it's not.