Expository lump is a fatal flaw for three reasons:
First, the forward action of the story stops dead while the author speaks to the audience. The explanation interrupts the action and defuses whatever tension has been carefully built up to now. By and large, your audience paid for a story, not an essay, so when the story stops, they stop reading. Even if you break these asides into smaller bite-sized bits, rather than a three-page essay, you're still serving lumpy gravy.
Second, expository violates the character's point of view. It's often the author talking, not the character, because the character already knows the information s/he is "thinking" and it doesn't make sense for the character to be thinking about that now. If the ninja jumps out at our hero, he doesn't stop and remind himself, "ninjas originally appeared in the 15th century during the Sengoku period of Japan but were considered dishonorable because of their use of covert methods" he's too busy thinking, "Look out!" As soon as the author starts explaining background history, we're no longer seeing through the character's eyes, and we've fallen out of the story.
Third, most of the information presented in expository lump is unnecessary. If the story is about the heroine hanging off a cliff, do we really need to know her fear of heights came from that time she fell off the roof when she was six? Why spend time telling us about another scene from another book when all one has to do is show us she's afraid? The answer is usually that the author has spent hours developing their setting, character, plot and so on and having put in all that work, feels compelled to tell the audience all about it. But it's redundant, off message, a distraction . . . bloating.
Cut the expository lumps out of your writing, and you automatically move it up to the next level.