Softbank's investing "strategy" has always been a mystery to me. Lengthy investment in perpetual money-loser Sprint, massive spending on the WeWork pyramid scheme, basically everything they do looks like either a sucker chasing an obviously bad bet or throwing good money after bad on a failing bet.
As for why now, that's easier--they just lost an absolutely ungodly amount of money on Sprint, WeWork, and various other catastrophically bad investments and are desperate for cash, so they're selling off things that are actually still worth something.