Just some "thoughts out of season." First, ...
Now, some hypotheses.
1. The original gospel pericope for Palm Sunday was something like John 11:47-54; 12:1-18:
11 47 Then the chief priests and the Pharisees called a meeting of the Sanhedrin. “What are we accomplishing?” they asked. “Here is this man performing many signs [signa]. 48 If we let him go on like this, everyone will believe in him, and then the Romans will come and take away both our temple and our nation.” 49 Then one of them, named Caiaphas, who was high priest that year, spoke up, “You know nothing at all! 50 You do not realize that it is better for you that one man die for the people than that the whole nation perish.” 51 He did not say this on his own, but as high priest that year he prophesied that Jesus would die for the Jewish nation, 52 and not only for that nation but also for the scattered children of God, to bring them together and make them one. 53 So from that day on they plotted to take his life. 54 Therefore Jesus no longer moved about publicly among the people of Judea. Instead he withdrew to a region near the wilderness, to a village called Ephraim, where he stayed with his disciples.
12 1 Six days before the Passover, Jesus came to Bethany, where Lazarus lived, whom Jesus had raised from the dead. 2 Here a dinner was given in Jesus’ honor. Martha served, while Lazarus was among those reclining at the table with him. 3 Then Mary took about a pint of pure nard, an expensive perfume; she poured it on Jesus’ feet and wiped his feet with her hair. And the house was filled with the fragrance of the perfume. 4 But one of his disciples, Judas Iscariot, who was later to betray him, objected, 5 “Why wasn’t this perfume sold and the money given to the poor? It was worth a year’s wages.” 6 He did not say this because he cared about the poor but because he was a thief; as keeper of the money bag, he used to help himself to what was put into it. 7 “Leave her alone,” Jesus replied. “It was intended that she should save this perfume for the day of my burial. 8 You will always have the poor among you, but you will not always have me.”
9 Meanwhile a large crowd of Jews found out that Jesus was there and came, not only because of him but also to see Lazarus, whom he had raised from the dead. 10 So the chief priests made plans to kill Lazarus as well, 11 for on account of him many of the Jews were going over to Jesus and believing in him. 12 The next day the great crowd that had come for the festival heard that Jesus was on his way to Jerusalem. 13 They took palm branches and went out to meet him, shouting,“Hosanna!”14 Jesus found a young donkey and sat on it, as it is written:
“Blessed is he who comes in the name of the Lord!”
“Blessed is the king of Israel!”
15 “Do not be afraid, Daughter Zion;16 At first his disciples did not understand all this. Only after Jesus was glorified did they realize that these things had been written about him and that these things had been done to him. 17 Now the crowd that was with him when he called Lazarus from the tomb and raised him from the dead continued to spread the word. 18 Many people, because they had heard that he had performed this sign [signum], went out to meet him.
see, your king is coming,
seated on a donkey’s colt.”
2. Then, someone decided to replace Turba multa with Cum approprinquasset (Matthew 21: 1-9), despite the fact that "palms" are to be found nowhere therein.
3. Ultimately, the reading of the Passion came to take center stage. As a result, the former mass was simply truncated and attached to it, at the beginning, as the blessing of the palms.
RESPONSORY ¤ John 11. 47-50, 53: Collegerunt pontifices et pharisaei concilium, et dixerunt: Quid facimus, quia hic homo multa signa fecit? Si dimittimus eum sic, omnes credent in eum: * Et venient Romani, et tollent nostrum locum et gentem. V.: Unus autem ex illis, Caiphas nomine, cum esset pontifex anni illius, prophetavit dicens: Expedit vobis, ut unus moriatur homo pro populo, et non tota gens pereat. Ab illo ergo die cogitaverunt interficere eum, dicentes: * Et venient . . .
Ant. ¤ John 12. 11 et alia: Cum audisset populus, quia Iesus venit Ierosolymam, acceperunt ramos palmarum: et exierunt ei obviam, et clamabant pueri, dicentes: Hic est, qui venturus est in salutem populi. Hic est salus nostra, et redempio Israel. Quantus est iste, cui Throni et Dominationes occurrunt! Noli timere, filia Sion: ecce Rex tuus venit tibi, sedens super pullum asinae: sicut scriptum est: Salve Rex, Fabricator mundi, qui venisti redimere nos.
Ant. ¤ John 12. 1 et alia: Ante sex dies solemnis Paschae, quando venit Dominus in civitatem Ierusalem, occurrerunt ei pueri: et in manibus portabunt ramos palmarum, et clamabant voce magna, dicentes: Hosanna in excelsis: benedictus, qui venisti in multitudine misericordiae tuae: Hosanna in excelsis.
Ant. ¤ John 12. 12 et alia: Turba multa, quae convenerat ad diem festum, clamabat Domino: Benedictus qui venit in Nomine Domini: Hosanna in excelsis.
RESPONSORY ¤ John 12. 13 et alia: Ingrediente Domino in sanctam civitatem, Hebraeorum pueri resurrectionem vitae pronuntiantes, * Cum ramis palmarum: Hosanna, clamabunt, in excelsis. V.: Cum audisset populus, quod Iesus veniret Ierosolymam, exierunt obviam et. * Cum ramis . . .
5. Only the last two elements just listed survive in the synthetic replacement of 1955. Liturgical revision obscures, rather than clarifies, origins.
32. In dominica Palmarum
Joa., XII, 1-24: Maria unguit pedes Jesu. Turba multa ... acceperunt ramos.
20. Missa in symboli traditione
Joa., XII, 1-8, 12-16: Maria unguit pedes Jesu.
26. In simbuli traditione missa prima
Joa., XII, 1-50 = L.32, B.20
80. Dom. in Ramis oliv., ad Sem Laurentium
Joa., XII, 12-13: Turba multa ... acceperunt ramos olivarum.
81. Item missa postquam veniunt ad ecclesiam
Joa, XI, 55-XII, 11: Maria unguit pedes Jesu.
- Yet in Würzburg, Murbach, and the Roman system:
St. Matthew's Passion