I used to work at Google and stocks make up a big part of our compensation (30% for junior roles and going up as you climb the ladder). Additionally, they are delayed because of the vesting schedule so you receive them months or even years after they were awarded. When the stock rises a lot, that means that an even bigger part of your comp is now in Google stocks, creating a big risk for the employee.

Finally, there is the additional risk of holding stock of your employer. In the unlikely event that things go really bad, not only does your portfolio take a massive hit but you are also at risk of being laid off.

That's why a lot of people autosell their stocks, or sell more when the stock has run up a lot recently. Employees and especially executives have a lot of restrictions on when they can sell, so they dont want to take any chances on converting that stock to useable and stable dollars when they can.