Honestly this has been a long time coming. Yes this happens more when the execs get huge numbers of stock vs cash because its something that can easily multiply to be many times their cash salary, but that isnt the only reason.
We all know that Symantec has been mismanaged for some time. The truth is that hard decisions needed to be made to get back to being more profitable, and without that, we all got put in this position. It is Hill's and Starboard's fault that we are here? No. They simply took advantage of a situation that they could and are making a ton of money and laughing all the way to the bank.
TBH I find it to be unethical (to sell of the company and then buy massive amounts of stock, his contract is also complete BS) and completely devoid of all semblance of humanity, but that is a different discussion.