Reason Behind This Ugly Turn in Relationship

Relationship took a beating in 2015, when India first got blamed for interfering in the Constitution drafting in Nepal.
India was also blamed for an “unofficial blockade” in the Madhesis issue, this generated widespread resentment against the country.
The politicians in Nepal exploited Nepali Nationalism and anti-Indianism successfully.
China seized the opportunity- In the past, China maintained a link with the Palace and its concerns were primarily related to keeping tabs on the Tibetan refugee community.
Now, that the monarchy is abolished, China has shifted its attention to the political parties, as well as institutions like the Army and Armed Police Force.
Today, China’s foreign policy is more assertive and China considers Nepal as an important element in its ever expanding footprint.