When water freezes, it expands. Note that this is the reason why icebergs and ice cubes float in water. So, when the water underground freezes, it expands and pushes outward (and up). This pushes up from the ground to the wall and can break or damage it. (This is also a big reason why cracks form in , rocks, sidewalks, and pavement during the winter and early spring.)
In "Mending Wall," the speaker believes, or wants to believe, that there is something in nature that "doesn't love a wall." In other words, he wants to believe that there is something in nature (i. e. the ground swells that break the wall down) that doesn't "love" the idea of walls and separation. The speaker similarly wants to believe that there is something in human nature that doesn't or shouldn't love walls and other literal and metaphorical ways we isolate ourselves from others.
The speaker's neighbor, on the other hand, thinks walls are useful, uttering the line:
Good fences make good neighbors.
Although it might seem obvious that the speaker is simply right, that it is bad when walls separate us physically and emotionally from others, the poem is not that simplistic. The speaker is just as stubborn as his neighbor who insists upon having the wall. The speaker has, perhaps, an unfair image of his neighbor: the speaker describes the neighbor as someone who "moves in darkness." And interestingly enough, it is the rebuilding of the wall that annually brings the speaker and his neighbor together: that which separates also reunites. This makes the phrase "Good fences make good neighbors" seem less hateful and more embracing. This would also imply that something "that doesn't love a wall" (frozen ground swells) is something that loves the opportunity for neighbors to get together because the broken wall provides that opportunity.