The roots of concerns are in the aggressive way that Chinese actors conducted cyber espionage and data theft in recent years.
At its heart were different conceptions of what constituted legitimate national security interests as whilst the U.S. and others conducted cyber espionage to understand China’s military capabilities, China was stealing intellectual property and handing it on to private and state-owned companies. Whilst this appeared to decrease after the 2015 Xi-Obama agreement, as recently as December 2018, the U.S. was indicting Chinese actors for data thefts.