China sold those drones to the Yemeni Houthis - China has been strategically flooding the Middle East with cheap, advanced drones, but the US just wanna blames Iran because the US really, really wants a war with Iran, which suits China just fine.
As a retaliation for:
The US keep selling weapons to Taiwan
Detaining the Huawei CFO because she violated the US sanctions on Iran - i.e., unilateral sanctions that were not recognized by China and that even the EU court had ruled were illegal.
Recruiting Uyghur from Xinjiang to fight in Syria since 2009, training them to be jihadists and then secretly sending them back into China to commit a series of escalating terrorist attacks between 2009 and 2016, and trying to build Islamic terrorist cells and networks in the mainland to destabilize China, (which ultimately led to the current mass crackdown in Xinjiang since 2016 as China decided to use a sledgehammer to put a stop to the CIA's plan;)
Planting and fueling the protests and riots in Hong Kong, (which was the last straw; actually, the US and the CIA might not be directly involved in Hong Kong - it's the NED, which is "loosely" connected to the US and actually does not directly report to the US, but China still blames the US nevertheless.)
My conjecture: China's strategy is not to get directly involved in the Middle East, but to indirectly support rebels and insurgents against the US, while making money at the same time. Basically, to use the exact same tactics the US had used to defeat the Soviet Union in Afghanistan. The US will have to commit more and more money and resources in the Middle East, which will ultimately bankrupt the US. That's the gist of it. Which is not a bad plan.