ARM was wholly owned by SoftBank. They did not have their own cash, it was all Softbank’s cash. And obviously SoftBank selling ARM means SoftBank wants to cash out, for whatever reason.

SoftBank made a ton of stupid investments like WeWork and was bleeding money. They needed to cash out their ARM investment to recoup major losses.

In fact, SoftBank is known for being the one of the stupidest big investors there is. They got lucky being an early investor in Alibaba, but have managed to piss most of that money away on getting left holding the bag on the peak.