Inner Mongolia Baotou, which accounts for nearly half of the world's rare earth output, posted in a filing with the Shanghai Stock Exchange that the halt was aimed at "balancing supply and demand." Prices of some rare earths have dipped dramatically in recent months and China has been known to limit rare earth production to drive up prices, so we'd assume this stoppage is not related to the simple laws of supply versus demand.
Nikkei reports that Baotou's shutdown is to test its market clout. A month-long halt will remove 5,000 metric tons of rare earths from the market and, as rare earth analyst Sam Berridge states, "We would expect a month-long shutdown from the largest producer in the world to impact prices reasonably quickly." All the more reason to start mining for rare earths locked beneath the Pacific Ocean if we're going to keep building advanced vehicles that need the materials.