Tesla stock is highly shorted and volatile.
Musk announced he's thinking of selling the company to a phantom private investor (who doesn't exist) for $420 and all stockholders will receive a $420/share buyout.
Obviously this has the effect of moving Tesla stock UP towards $420.
If he had a deal worked out and signed, then he could legally announce such a deal.
However, he doesn't have such a deal worked out, or signed, and the "private party" doesn't exist.
So it's wanton stock manipulation, which is totally illegal and he's going to get completely ass raped by the SEC for this, it's criminal. He'll probably go to jail.