Elon Musk has sold $9 billion worth of Tesla (TSLA  ) stock in the last two weeks, accounting for roughly 5% of his total stake in the company. That's half of the amount that Musk offered to sell in a poll posted to his Twitter (TWTR  ) account, but also far more than the amount analysts had predicted Musk would sell for tax purposes.

In the coming months, Musk is expected to face taxes of up to $15 billion on stock options, meaning that he was always expected to sell some of his stock to pay that bill. However, he is currently expected to sell far more than $15 million worth of stock.

Musk receives company shares as a part of his compensation plan as drafted in 2012. Rather than making a salary or bonuses, Musk gains wealth through stock price growth. In 2012, Musk was paid 22.8 million shares at $6.24 per share. On Monday, Nov. 23, Tesla shares closed at $1,156.87 per share, meaning Musk has made roughly $30 billion in stock gains.

Because they are considered compensation, Musk's stock options are taxed just like ordinary income, with a potential combined state and federal tax burden of 54%.

Musk is also able to use his shares as collateral in order to apply for loans, another expense that could hypothetically be covered by the sale of stock.

"If the price of our common stock were to decline substantially, Mr. Musk may be forced by one or more of the banking institutions to sell shares of Tesla common stock to satisfy his loan obligations if he could not do so through other means," Tesla wrote in its third-quarter filing with the Securities and Exchange Commission (SEC). "Any such sales could cause the price of our common stock to decline further."

Musk's own filing with the SEC stated that $1.1 billion in shares included in a total sale of $2.5 billion were sold to cover his tax bill.

"The shares of common stock were sold solely to satisfy the reporting person's tax withholding obligations related to the exercise of stock options," the November 8 filing reads.

However, Musk went on to sell far more stock, eventually dipping into his existing shares. He is expected to owe long-term capital gains taxes of $1.3 billion on the sale, meaning it doesn't make much sense to make such a sale in order to pay off a different tax bill.

"It wouldn't make sense from a tax perspective for him to use those proceeds for the options tax," Toby Johnston, a partner with the accounting, consulting, and wealth management firm of Moss Adams, told CNBC.

Musk went to Twitter once again to support these theories saying that "A careful observer would note" that his actions have been closer to tax maximization than minimization."

So far, theories as to what Musk plans to spend the proceeds from these sales on vary from SpaceX programs and private business ventures to funding for his livelihood.

"For people at his level, taxes aren't always the primary driver of investment decisions," said Johnston. "It still feels like there is a piece missing to the puzzle that we may not know about."