In meetings and at the office there are definite rules about delegating tasks. While delegating tasks or responsibilities to a subordinate is a straightforward matter, delegating tasks to a co-worker is somewhat more problematic. The problem is that no one really has the right to delegate tasks to a co-worker. If a task is to be given to a co-worker it has to be done in an agreed upon manner where both parties are aware of what is going on and both agree to the work being delegated. It is almost a dance being performed in a meeting when one person tries to delegate tasks to a co-worker. This is not the case however with email. For some reason, people think that they can just give something to someone else to do in an email when they would never consider delegating the same task in person. Email is adding an effective layer of anonymity to dealings between co-workers that does not exist in live meetings. This layer of anonymity is allowing people to get away with behavior that would just be unacceptable anywhere else at work. Unfortunately, it is through processes such as these that people come to feel that they have lost control of their working lives. When co-workers, people at the same level of the organization can assign tasks and responsibilities, one has lost another element of control and this in turn can lead to greater feelings of being overwhelmed.
Perhaps the most surprising thing about the anonymity that email has introduced is the tendency for people to try to delegate upwards, to assign responsibilities to their boss in ways they never would have considered doing before. Perhaps this is a reaction to being out of control and an attempt to re-assert control but I have experienced it myself and heard others complain of the practice as well. Certainly in the past you could have asked your boss to do something but you would have done this with some degree of trepidation and handled it in a formalistic manner so not do disturb the worker-boss relationship. With email however, those older social niceties are pushed aside in favor of directness and speed.