A typical expository paragraph starts with a controlling idea or claim, which it then explains, develops, or supports with evidence. Paragraph sprawl occurs when digressions are introduced into an otherwise focused and unified discussion. Digressions and deviations often come in the form of irrelevant details or shifts in focus.
Irrelevant Details :
When I was growing up, one of the places I enjoyed most was the cherry tree in the back yard. Behind the yard was an alley and then more houses. Every summer when the cherries began to ripen, I used to spend hours high in the tree, picking and eating the sweet, sun-warmed cherries. My mother always worried about my falling out of the tree, but I never did. But I had some competition for the cherries — flocks of birds that enjoyed them as much as I did and would perch all over the tree, devouring the fruit whenever I wasn’t there. I used to wonder why the grown-ups never ate any of the cherries; but actually when the birds and I had finished, there weren’t many left.
No sentence is completely irrelevant to the general topic of this paragraph (the cherry tree), but the sentences Behind the yard was an alley and then more houses and My mother always worried about my falling out of the tree, but I never did do not develop the specific idea in the first sentence: enjoyment of the cherry tree.