This excellent poem doesn't actually tell us what the event was that produced such a notable alteration in the behaviour between the speaker and the person the poem is addressed to. However, from the central simile that governs the poem, we can infer that some kind of argument has occurred between the two characters, which has now meant that there is a distance between them and a wary sense of impending danger. The speaker describes both himself and his audience as being like "the deer" who know are very careful and tense, ready for any sign of danger and always ready to flee:
who walk in single file
with heads high
with ears forward
with eyes watchful
with hooves always placed on firm ground
in whose limbs there is latent flight.
The last line of this poem is particularly powerful, as your question suggests, by the reference to "latent flight." The speaker deliberately chooses this image to end his poem, expressing the constant readiness that both he and his audience now have to flee any sign of impending danger or threat. Presumably we can infer that after some kind of argument, this couple has now lost all sense of ease and expect some kind of renewal of anger or frustration, which they are constantly ready for. The simile, reinforced by the last line, presents them as being very skittish and ready to fly when necessary. All sense of trust has been lost.