I don't understand why this suddenly happened (except if asked by the USG in response to the recent scare/reality over rare earths).

The 50% ownership by a sanctioned entity was a reality for a while, and was an issue as soon as the purchase. This didn't change recently. So, this action should have been part of the pre-purchase review (CFIUS in the US...I assume there is an equivalent in China). On the face of it, this all could have been avoided by having a non-sanctioned entity (including another random Chinese company) own enough of the company to get sanctioned entity ownership below 50%.