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It's market manipulation, Elon writes a tweet and the fans go out to buy or, as in this case, sell. He sells a bit of BTC and surely taking advantage of the falling price he buys some more.

Definitely possible and something that crossed my mind.

@krishool do you believe this to be solely based on environmental concern?
People are saying that he's trying to game the system. There are insinuations that he might have sold the coin he bought.
I don't know what to believe.