The Peter Pile-OnThe Peter Principle posits that in a hierarchical organization, employees will tend to be promoted to their level of incompetence. When you're performing your job well, you will be recognized as competent and be rewarded with a promotion. You will continue getting promotions until you reach a point where you are incompetent, at which point the promotions stop. But because it's often difficult to demote or fire people (especially in bureaucracies), that's where you'll remain.
I wish to propose a cousin to the Peter Principle that may apply even in non-hierarchical organizations. I'm not good at the naming thing, but until I hear a better label, I'll dub it the "Peter Pile-On." The idea is that an employee will tend to acquire additional duties until he performs none of his duties competently. As long as you're performing well, others will consider it a wise decision to burden you with more responsibilities. The process will not stop until you're performing your current duties poorly.