It is indeed strange, why China abandoned a successful foreign policy in favour of a confrontational one. Ideology may provide an answer.

From Deng Xiao Ping's "cat period" until 2013, pragmatism overruled ideology.

Right after Lehman it became consensus, that China must differentiate itself in many aspects and, broadly speaking, put ideology ahead of pragmatism.

So in 2013 previous principle was reversed, Maoist "struggle" became an overarching term, and that includes foreign policy and related activity.

Here I become a bit speculative. In spite of (at that time vice president) Xi Jin Ping's claim in 2011, that China will never export revolution, fundamental realisation seems to be, that socialism can only work if it is global (a Leon Trotsky postulate), which implies the need to stand up for proletarians everywhere and to support their "struggle".

Why: Socialism is a road to communism. Communism is also defined by absence of traditional government, that by default must include absence of traditional world order. If you are the only country without government, then you must fail.

So I speculate, that at least exporting "struggle" has become a foreign policy task. Hence aggression, to confront the class enemy.