From The Gospel of Matthew, by Curtis Mitch and Edward Sri, commenting on Matthew 21:1-11
As they near the end of their procession into Jerusalem, the crowds also fittingly recite words from Ps 118:25–26, a pilgrimage hymn typically chanted on the way to the temple for the major feasts. Hosanna is a Greek transliteration of a Hebrew word meaning “Save us,” which became an expression of praise in liturgical worship. The blessing on the one who comes in the name of the Lord was customarily invoked on pilgrims coming to the temple, but here in reference to Jesus it takes on christological significance. Jesus is the one who, coming in the Lord’s name, represents God and acts on his behalf (see 23:39).
The city’s cold response to Jesus’ arrival stands in contrast to the crowds accompanying him. As at Christ’s birth, when “all Jerusalem” was “greatly troubled” by news of a new “king of the Jews” (2:2–3), so now the whole city was shaken by the uproar surrounding Jesus’ regal procession into the city. The way the pilgrims acclaimed Jesus as king would have been troubling for the Roman authorities in Jerusalem, who would see any rival king to Caesar as a threat.
The priestly leaders of the temple also would have been alarmed by a man from Galilee claiming to be king, since they would have seen this popular leader from the north as a threat to their own positions of influence in Jerusalem.
Consequently, instead of rejoicing in the king’s arrival (as Zech 9:9 exhorted them to do), the Jerusalem inhabitants question Jesus’ worthiness to receive such a royal welcome: Who is this? they ask. The crowds (probably consisting of many Galilean Passover pilgrims) emphasize that Jesus is one of their own, not a Judean. He is Jesus of Nazareth (not Jerusalem), a city in Galilee (not Judea).