It's pretty clear what's driving the China's nuclear buildup. The changing geopolitical situation. In the last half a decade alone there's been a huge shift in how the US perceives China. Long time ago, it used to perceive it as a partner and part of the global economic system that plays into the US hand. But since then, the US changed its opinion and for several years now has been attacking China via economic, monetary, technological, political and media means, using its own might in those areas, as well as urging/making its allies to do the same.
Years ago, when US and Chinese economies were pretty intertwined, a war between the two would have been quite damaging to both. But if there comes a day when China will somehow for the most part be decoupled not just from the US economy but also from a lot of Western countries' economies - then a war between the two might not be as damaging. And in such a situation, actually using its huge numerical superiority in nukes, might not be so completely far fetched for the US, no matter the losses in such an exchange. Of course, if China manages to at least partially catch up in nuke numbers by then, then a nuclear exchange would be far more costly to the US.
Hence the Chinese urgent drive to build up their nuke arsenal.