I'm not necessarily a Musk fan, but it was (as I understand it) based on the percentage increase in their share price, something like 10% of the increase in the value to shareholders would go to him. So, for every dollar the shareholders gave him, they got 9 dollars (approximately). So they didn't expect it to be as big as it was, because the appreciation in Tesla's stock price was crazy big. Matt Levine of Bloomberg has written on this in more detail.