Ah, the plough (American spelling is "plow," in case you were confused). What's a mouse to do? You build a nest, insulate it and stock up against the coming winter, and then a plough comes and mows it over. The plough tearing up the mouse's nest is the opening situation that motivates the speaker of the poem to write at all, so you better believe the plough is doing something important. It's possible that the plough represents human civilization and the kind of "progress" that has "broken nature's social union," as the speaker says in line 8. What else might the plough represent?
Line 6: The speaker personifies the "pattle" (part of the plough) when he describes it as "murd'ring," since he'd be the one to "murder" the mouse, not the plough.
Line 7: The plough could be a metaphor for "man's dominion," or the rule of mankind in the world—it's a piece of man-made technology that tears up nature for man's benefit.
Line 29: The speaker uses onomatopoeia with the word "crash!"—because it sounds like what it means. He highlights the "crash!" sound with alliteration, or the repeated C sounds later in the line as he describes the noise of the "cruel coulter" (another part of the plough) as it passes over the mouse's nest.