When they had come near Jerusalem and had reached Bethphage, at the Mount of Olives, Jesus sent two disciples, saying to them, “Go into the village ahead of you, and immediately you will find a donkey tied, and a colt with her; untie them and bring them to me. If anyone says anything to you, just say this, ‘The Lord needs them.’ And he will send them immediately.” This took place to fulfill what had been spoken through the prophet, saying, “Tell the daughter of Zion: Look, your king is coming to you, humble, and mounted on a donkey, and on a colt, the foal of a donkey.” The disciples went and did as Jesus had directed them; they brought the donkey and the colt, and put their cloaks on them, and he sat on them.
But while every commentator will tell you what Matthew has done with Zechariah, few attempt to tell you why he has done it. In this book, I suggest a possible reason. Keep in mind that the folks at Qumran (collectors/authors of the Dead Sea Scrolls) believed in at least two messiahs: (1) a priestly messiah and (2) a royal messiah. The Qumranites seem to have used a super-fancy reading of Zechariah 4 to support this belief: "the two sons of anointing".
Could Matthew have been answering those who expected dual messianism by using Zechariah 9?