The volumes on options are not all that large typically. So many people will put a sell to open at a high price to see if somebody really wants the option.. sometimes people will bite and it causes a big jump in the option price.. once that happens it starts other people jumping in as well (and algorithms). But you can sometimes see options that have volumes of less than 10 contracts in a day -- sometimes 0 for non popular stocks. So if softbank was buying big quantities daily that could easily run up the price. And the wild thing is that if you see a big jump in call option interest way out of the money, then people who are wondering about the underlying stock think, hmm, maybe people are aware of some upside on this stock.. I think I'll buy some more shares of tesla or whatever because the call option behavior is so bullish. So basically if you want a stock to move one direction or another you probably could manipulate most easily by buying large quantities of options on one side or the other in such a way that people think you know something that you don't.
The options market is relatively small and many of the market participants are trading on the greeks, basically betting on what the rest of the market is doing rather than on the fundamentals of the underlying economic activities, so it's easier to trigger a thundering herd with options activity.
