Four technical centers in China that are associated with the Tianhe-2, the world's most powerful supercomputer, were put on a list of restrictions by the U.S. government. According to the WSJ, the reason given is because these centers are acting "contrary to U.S. national security or foreign-policy interests."
The Tianhe-2 and the Tianhe-1A are both "believed to be used in nuclear explosive activities," according to a notice posted by the U.S. Commerce Department on February 18. However, designers for the Tianhe-2 said that the supercomputer is used for scientific research such as genome research.
Considering it's well known that China already has and continues to develop nuclear weapons as well as nuclear reactors, the reason given sounds quite strange. The Tianhe-2 supercomputer uses mainstream server chips from Intel, so it's hard to believe China couldn't find another way to gets its hands on those chips without having the four technical centers directly involved in acquiring them.
Right now, the export restrictions seem to refer only to those four centers and not to China as a whole, potentially making a bypass of these restrictions very easy. The four technical centers that are related to the Tianhe-2 supercomputer are located in Changsha, Guangzhou and Tianjin, and it also includes the National University of Defense Technology in Changsha.
The real reason could be that China has also increased its restrictions and demands for U.S. companies lately. One of the more recent demands is requiring companies to allow access to their software source code in order for the Chinese government to verify it for backdoors. The demand also includes adding ways to monitor their users and handing the government the private keys to encryption. U.S. companies, along with the White House, have decried the new requirements as invasive.
Horst Simon, a supercomputer expert and deputy director of Lawrence Berkeley National Laboratory, said that these restrictions could incentivize the Chinese to invest more heavily in developing their own chips locally.
Chinese companies already use architecture licenses for ARM or MIPS to build their own chips. Even if both ARM and Imagination were also banned by the UK government from licensing their architectures out to China, Chinese companies would still be able to keep the previous designs and continue to build on them.