Secondly, it's the employees who would have the incentive to leave, not the company to fire them. Assuming a typical 4 year vesting period, it's right at this year when their compensation would have _dropped precipitously_.
Thirdly, the second individual had been at Twitter for more than 4 years, and unless they left and came back (doesn't look like they did), they'd still be vesting their original equity grant. The amount of time they've been at the company isn't even common between the two, so I don't see why you'd use it to establish a theory.