Unrealized gains, unrealized losses... I'm not going to say they're totally meaningless, but they sure don't tell the whole story. Granted people can use them as collateral for loans, but tens of billions one way or the other doesn't really impact Musk in any material way.
Having said that I still think that Tesla is insanely overvalued.
I tried to find an explanation for the drop. Best I could do was this from Reuters:
"Tesla shares tumbled 9.74%, its biggest one-day percentage drop since April 20, after the electric-vehicle maker reported a drop in its second-quarter gross margins to a four-year low and CEO Elon Musk hinted at more price cuts."
Having said that I still think that Tesla is insanely overvalued.