Some of my guesses.
China is upset that India has rejected the BRI or OBOR (CPEC is a part of it).
China is upset that India said in the parliament that POK and COK are an integral part of India. Remember "Jaan de denge" speech of Amit Shah?
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China is upset of Ladakh becoming a UT.
China is upset about Darbok Shyok DBO road. Gives India speed and height advantage. Also 61 roads with branches to LAC. India's highest bridge done recently.
China is upset due to landing of advanced helicopters on Siachen.
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China is upset that India is preventing the ~400 per year "Walk ins" that Communist army was used to, on the LAC.
China is upset of India joining Wuhan origin virus probe call.
China is upset that India is wooing 100s of companies which are wanting to leave China.
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China is upset tht India has successfully thwarted Pakistani terror (recent Pulwama 2 IED bomb plan busting).
China is upset about Vocal about Local in India. Making threats regularly against business disruption.
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China is upset that India stood up for landlocked Bhutan recently, and helped Bhutan retain Doklam plateau, a Bhutanese territory.
Can you think of more reasons as to why China is miffed with India now?