Typically, bonuses are calculated upon company profitability and individual performance. Most companies have calendar fiscal years and in general, bonus pay-outs occur as early as December and as late as April. If someone works for nearly a year at a company, the bonus is a compelling reason to stay and collect the money. When hiring, many companies pro-rate the new employee's first year bonus. The result is that departing employees race out the door once bonuses are released, and incoming employees try to start as early in the year as possible.
Some companies wait several months past year's end before paying bonuses.
In addition, it allows an incoming employee to receive only a partial or pro-rated bonus from the new employer. When someone does leave well into the fiscal year, months of work were performed which are free of bonus compensation and save the company money. The actual policy behind a pro-rated bonus will vary. Some examples are a proportion of the full bonus based upon time served or a mid-year cutoff for full bonus. To the uninitiated, all this might seem needlessly complex.