You need to have read Palm Sunday (1).
The Pius V liturgy for Palm Sunday was accounted for by Fr Thurston in a neat CTS booklet. He was a more elegant writer than I am; and more learned. But I think he probably got it wrong.
He explained the S Pius V Palm Sunday in this way:
The preliminary rite for blessing the Palms consists of the remains of what was originally a separate Mass. It includes all of the components of a Mass ... even a Preface ... but not the Consecration and Oblation. What clearly happened originally was that clergy and people attended one Mass at a church outside town; then progressed into town for a second Mass.
I think that, over the years, many of us have come uneasily to feel that, logically, two possibilities are equally probable:
(1) Thurston's: we have here the eviscerated remains of what was originally a full Mass; or
(2) the Blessing and distribution of the Palms was gradually built up by accretion, with the structure of the Mass providing a pattern.
I think the second of these models deserves a run for its money. But I want to look at the 'Preface' (translated below mostly by O'Connell/Finberg):
It is very meet, right ... Lord, Holy Father, Almighty everlasting God: whose glory is in the wisdom of thy Saints. For to thee thy creatures render service, acknowledging thee as their sole origin and their God; and the entire fabric of the universe joins in praising thee; and thy saints bless thee. For they boldly proclaim that great Name of thine only-begotten Son before the kings and powers of this age. Around him stand angels and archangels ...
I don't actually think this is a superb piece of Latin. I would be surprised if it had been composed by S Leo I or even Leo XIII. But its content is very good dogma. And it is attractively cheerful.
We are blessing branches ... or a branch ... of Olive ... or perhaps of Palm. And we regard the sanctification of these inanimate parts of creation as a sign and foretaste (some 'biblical scholars'might use their fancy grecism 'prolepsis) of the restoration of that creation which fell with and through the primeval Fall (Romans 8:18-24; this is worth reading). These blessed twigs will indeed (see the next prayer) be a 'tuae gratiae sacramentum'.
And so, as the King rides past on his donkey, Creation (omnis factura tua) comes to life (is this a bit Narnian?) and joins in praising him (collaudat). But the Saints are busily blessing too, and speaking with parrhesia before the earthly powers. And ... get this ... not only the Saints but the angels and archangels join the praise.
So it is eschatological: we are teetering here on the edge of the great Restoration at the End when all shall have been put right, even in the trees along the sides of the roads. They are already praising their Maker, and it's not surprising that the Saints get caught up in this cosmic glorification. And ... Yes! ... the heavenly powers, unfallen, seeing this apokatastasis have gathered around the Only-begotten and are singing for all they are worth.