Think about it. You're tired of your job and interested in making a change. It's, say, August. You know that if you wait until the end of January you'll get your annual cash bonus, say, $30K. Also, between August and the end of January you get a few stock vesting periods which amounts to, say, another $50K. So you can leave now, or you can stick it out for a few more months and get $80K to help smooth your transition.

What do you think you'd do? If you're the sort who doesn't need money and only works for the challenge and the chance to contribute to something important, you would probably just leave. If your paychecks actually matter to you, the $80K (plus whatever your salary is during the interval), will probably make you decide that waiting a bit is a good idea.

Did you read the article? Bonuses that they only receive if they stay for a certain period of time.