The entry into Jerusalem happened on the first day of Passover week, when the city's population swelled from approximately 40,000 to somewhere between 200,000 and 2 million pilgrims, depending on which ancient estimate you use. Josephus, the first-century Jewish historian, gives numbers that modern scholars consider inflated, but the point stands: Passover was when Israel remembered the Exodus, the original act of divine political liberation. The Romans added extra troops to Jerusalem during Passover specifically because the combination of nationalistic memory, religious fervor, and massive crowds made it the most volatile week of the year. Any messianic demonstration during Passover week would have been read — by everyone — against that backdrop.
The crowd's actions were not spontaneous. John 12:12-13 says "a great multitude" that had come to the feast heard Jesus was coming and went out to meet him. They took palm branches — a detail worth stopping on. Palm branches were the symbol of the Maccabean revolt, the most recent and celebrated act of Jewish military resistance, only about two centuries earlier. When Simon Maccabaeus entered Jerusalem in triumph in 141 BC after expelling the Syrians, the crowd carried palm branches. The gesture carried the same symbolic weight that a flag carries today: it announced expectation of military victory.
Zechariah 9:9 is the prophecy all four gospels see fulfilled in the entry: "thy King cometh unto thee, meek, and sitting upon an ass, and upon a colt the foal of an ass." Jesus arranged the animal in advance. He chose the donkey deliberately against the available option of a horse — the war animal, the animal that would have fit the crowd's expectation perfectly. He was answering Zechariah, not the crowd.
Commentary is from a charismatic Protestant perspective, drawing on KJV text and public-domain sources including Spurgeon, Andrew Murray, and Matthew Henry.