I'd say this was the main reason. The original employees had already proven they could manage and grow the organization from 0->even, and hadn't slowed down expecting to move the company from even->profit.
Now they have "supervisors" who don't know anything about the business or technology, shoved in on top of them, who's only role is to act as liaison between them and the corporate officers.
Bristling at the lack of corporate loyalty shown to the people that made the company happen, those people leave. The calculus is really very simple.
I've seen this pattern happen a number of times, and the usual result is that these new managers show up, make poor decisions (because they don't understand the business) cycle out (taking their stock with them), repeat 4-5 times until a manager shows up who is so inoffensive that the remaining engineers simply end up running the show in the end anyways and the manager just shows up to weekly meetings so she/he can liaise with corporate.
In the meantime, the company stock has diluted as it fled out the door to the past managers, the company lost most of its senior engineering staff, forcing them to promote lower level, less capable staff to senior positions or to hire externally (again people who don't understand the business further exacerbating the vicious cycle).