Softer be they than slippered sleep the lean lithe deer the fleet flown deer.
Ah, alliteration, our old friend. Notice how all of the alliteration clusters around the deer? We're focusing (again) on how lovely and fast and pleasant the deer seem to be. And if the structure of these lines looks similar, that's because it is—you've seen exactly this structure in lines 6-8, which were also talking about deer.
Since we've talked a little bit about alliteration and repetition in lines 6-8, and since that pattern is holding pretty steady, we wanted to mention one other thing about the deer (since, after all, Cummings' speaker seems to have a whole bunch to say about them).
Notice how the lines of the poem get shorter whenever we get to descriptions of the deer? Check it out: count how many syllables there are in, say, line 15. (We count eight.) Now count 'em in line 18. We count… four. And line 17? Four again. What's up with that? Well, we've got plenty to say about that in "Form and Meter," so don't you worry.
For now, though, we just wanted to point out that the deer descriptions seem to be condensing slightly. Heck, even the words are all monosyllabic. It's as if the poem is getting down to the bare bones whenever the deer enter the picture.