Tesla’s share price fell 8.6% on Monday and has seen values fall at least 15% this week. The drop, which analysts report is at least partly down to CEO Elon Musk’s actions, has seen his personal value fall by some $15 billion.

Monday’s stock price drop was the sharpest one-day fall since September and follows Musk’s comments on Twitter saying that the prices of cryptocurrencies “do seem high.” That’s particularly concerning to investors since Tesla publicly announced that it was investing $1.5 billion in the Bitcoin cryptocurrency.

The decision to invest in the currency was not unilaterally his, the CEO has said. Tesla, through Musk, explained that the investment was made to “provide us with more flexibility to further diversify and maximize returns.”

Read More: Musk’s Tesla Says It May Accept Bitcoin As Payment After Investing $1.5 Billion In Cryptocurrency

The fault may not be all Musk’s, though. On Tuesday, Bloomberg reported that Oliver Zipse, BMW’s CEO, threw Tesla’s position as a long-term EV leader into question.

“It won’t be easy for Tesla to continue at that speed because the rest of the industry is moving ahead big time,” Zipse told the outlet.

Dan Ives of Wedbush Securities, meanwhile, told Yahoo Finance that the sell-off may also have infuenced Tesla’s decision to discontinue the cheapest Model Y trim as well its wider decision to attach itself to Bitcoin.

Indeed, the cryptocurrency’s price is down, too, this week following Musk’s statements. Tesla’s decision to invest in it, though, did increase prices substantially last week.

Whatever the case, Tesla’s stock fell to $672 per share this morning before markets opened. That’s lower than it was when it entered public trading on the S&P 500 in December 2020. The stock is now down 1.3% on the year, compared to the S&P 500’s 3.2% gain.