You really need to have a line company infantryman elaborate on this for you, Judy. However, in case one does not do so here, I'll give you my thoughts, as influenced by my reading and interviews of veterans.
In this context, "rear echelon" is, indeed, a pejorative term. The lives of front line troops are always at risk. Generally speaking, the further you get from the front, the less this is so. Troops on the front line have a tendency to disparage those who are not taking the same chances with their mortality. Also, certain items (good food, warm clothing, etc) occasionally get taken up as they work their way through supply lines toward the front and, from time to time, don't end up where they were intended to go. This sort of thing tends not to endear those in the rear to those in the front. Who, exactly, is "rear echelon" varies, somewhat, according to whom you are talking. For instance, in "Citizen Soldier" Ambrose quotes a soldier as expressing the opinion that rear echelon was defined as anyone behind his foxhole.
We'll see if anyone else can expand on the matter.