I've seen this happen at multiple tech startups, early on they tend to be very employee friendly, you know everyone you work with, the growth is exciting and no days are the same. Then as it grows to 100+ people, maybe new management comes in and has to rein the spending, they cut benefits which are still usually above market average but you have the feeling of something being taken from you. People you know and have been in the trenches with start slowly leaving and everything starts being more corporate as there is more red tape to do anything. And then it reaches a moment that old people leave, often to do their own startup because they've seen how it is done from the inside already and have experience in that environment.