"Bells for John Whitside's Daughter" is a classic example of an elegy, a poem mourning a death. This elegy centers on a great contrast: the differnece in how the little girl acted while she was alive, and her stillness in death. In her life, the girl is extremely active, as she fights imaginary "wars" and charges the pond in her backyard; she even causes movement around her through the "scuttl[ing]" geese who are running from her. Ransom describes her as a "tireless heart" because of all the movement and "speed" she embodies.
In contrast, in death, the little girl does not move at all. Significantly, she is described as "primly propped," indicating that she has been acted on (someone else propped her up), rather than controlling the action herself as in the rest of the poem. Most importantly, Ransom does not ever say that she is dead; rather, he describes her as being in a "brown study," which is a phrase for a state of daydreaming or deep thought.
As a result, death in "Bells for John Whitside's Daughter" seems absolutely unnatural, especially for this little girl, who was so active in life. The mourners are "vexed" and "astonished" at the way death has changed her natural way of being.