This is when low performing employees are rewarded at the expense of high performing workers. I have seen it a million times in a million different iterations, so I will write again of the pitfalls of letting high performers take on the lion's share of the workload.
When you have a team of employees, you inevitably manage a situation where certain people are much more productive than others. For whatever reason, you have your stars and you have your bench warmers. This is just the way of it.
Without thinking about it, managers tend to put more work on the high producers while taking work away from those who can't seem to handle it. In essence, the workers that show the most initiative get punished with an increased workload while the low producing employees work under lowered expectations and ultimately have more free time and less stress.
It is understandable. Managers are very often doing some form of extreme multi-tasking with at least twenty projects and tasks every day, and dealing with personnel issues is not exactly a tantalizing concept. It is easier to give more work to your employees you can depend on than to work with the weaker employees with whatever problems they are struggling with. It is simple math, but it is bad math, and here is why: When you do this, you are keeping your weak employees weak and you are crushing the souls of your high performers.
Ultimately, by not working to improve the skills of your low performers you are creating an environment of resentment, and in the end you lose your high performers to other companies while your low performing employees hang on for decades. In the long term, this practice costs you more time and money (in turn over and loss of productivity) than the time and money you might have spent working with your weaker employees.
What you are communicating (yes, it always comes back to communication) is that you are willing to sacrifice the well being of your high performers so that your weaker employees can maintain the status quo, and while you might not mean to be communicating that, while you might not have a clue that this is the message that you are sending, it is the message that is being received, and it can destroy your team and your productivity.
Managers must be accountable to their employees, and this is an important but often overlooked method of exemplifying accountability. By showing awareness and equitably distributing the workload, all boats rise together, weaker employees gain new skills, and your high producers don't get burnt out because they are laboring under the workload of three people. Your employees will trust you to do what is right for the team and the company, not what is easiest for you, and will in the end work harder for you.