As I've pointed out many times before, employers have more bargaining power than employees. The primary reason for the unequal bargaining power is that employees vastly outnumber employers.
But there are exceptions. Take baseball players, for example. Suppose there are five catchers who are the best in baseball, and there are no other good substitutes for their elite catching abilities. Because there are only five best catchers but thirty baseball teams, here is the unusual case where employers vastly outnumber the employees. What winds up happening is that the employees make huge salaries becaue they have more bargaining power.
I should point out that this is as unjust an economic result as when the employer has more power. In the normal case, the employer gets to make profits off of the employees' ability to produce economic value. In baseball, the opposite is the case. The reason why baseball makes so much money is not because of the players, but because of the guys who invented the sport a very long time ago and because of the huge amount of marketing capital and physical capital (in the form of baseball stadiums) invested into the sport. The best catcher or best first baseman doesn't deserve to make any more money than the best muggle-ball player. Muggle-ball would in fact be a much more entertaining sport, but no one has yet invented it and invested billions of dollars of capital into it.
Movie stars also seem to have more bargaining power than their employers. I often read the complaint that movies aren't "profitable." In fact they are very profitable for the movie stars. Every big budget movie that has "lost money" has actually made a lot of money for the highly paid movie stars. This is just another example of how most of an industry's profits go to the party with the most bargaining power. Nearly always that means profits disproportionately go to employers, but that's not always the case.