A ballad is usually a song. When poetry takes on ballad meter, chances are that we're going to be reading something that hearkens back to the days when bards roamed the land singing stories over campfires and in king's halls. Since this poem is set in the days of myth and legend, the ballad meter is a good way to conjure up a little bit more authentic antiquity.
The ballad is a pretty regular old form, with four iambs—that's eight syllables per line. Cummings, however, has other things in mind. Sure, there are usually eight syllables per line in this poem, but instead of four iambs, there are four trochees—that's a combination of two syllables that start with an accented syllable and then move to an unaccented one: DUMda. (Say "elbow" out loud and you should hear a trochee in action.) Check out line 1 for examples of these guys:
All in green my love went riding.
And that's just the start of where things get weird. See, sometimes (usually whenever violence gets mentioned) Cummings shortens the line to seven syllables—like, say, in the line
the cruel bugle rang before.
So, what's up with these metrical choices? Well, we see the trochaic inversion (using trochees in place of iambs) of the conventional ballad form as really in keeping with the way that this poem's content (speaker is killed by his beloved, at least in a metaphor) presents the reverse of what a typical ballad would have (speaker loves and is loved in return). Cummings is flipping the typical ballad on its head here, even on a rhythmic level.
And those seven-syllable lines also make sense, especially given the violent content they accompany. The beat naturally stumbles when we sense (or are told about) the impending doom of the deer (who we come to find out is the speaker). This isn't smooth riding for him at all.
The poem's form is also incredibly regular and also sorta weird: there are repeating patterns of three-line and two-line stanzas. Each two-line stanza begins with four animals: hounds, stags, roebuck, or does. While of course this advances the backdrop of the hunting narrative, this structure seems to be a purposeful choice, given its consistency.
So what's behind the choice? Well, if you think about it, these two-line stanzas are in some sense a return to the action of the hunt. The three-line stanzas tend to be more descriptive, and with less action verbs. In that way, then, the shorter stanzas are how Cummings communicates the metaphor of the relationship (hunt = love affair). Nature, then, becomes a way to reflect, or double, the condition of the human characters (the speaker and his lover). That couple, doubled, seems to produce animals in fours, yet which are represented in couplets. In a strange way, the math actually works. Pretty neat, eh?
As well, and here's where things get even stranger (if that's possible), whenever we arrive at a stanza discussing the deer, we have two four-syllable stanzas instead of one eight-syllable stanza. Check out lines 7 and 8, for instance, or lines 27-28. The typical eight-syllable line expands into two lines, which helps to draw attention to the deer. And since it tuns out that the deer-hart is actually one big long metaphor for the heart of the speaker, it makes sense that our speaker would want to dwell on deer as a central image. After all, who doesn't like talking about themselves?
In weird, wild, wacky ways, then, this poem's form and meter double back to highlight the content of the speaker's experiences as he's stalked by his lover in this metaphorical hunt.