A king in disguise is a hero. They can believe it the moment they hear the braying of the donkey, because a king in disguise doesn’t come riding on a white horse (that job is left to the knights, who can freely boast).
So instead of gold, they pave his path with branches from palm trees, because a hero is always victorious. And with their tongues, they cry to the hero to rescue them, because they’ve been waiting too long and the rumours are too numerous to ignore.
They don’t know that he’s actually a hero in disguise. He will do things that a hero will never do, and say things that most people don’t like to hear, and there’s never been a hero like him. He will be hated by fake heroes, so he can remain incognito until the right time comes.
And when the right time comes and goes, only some will realise that a hero in disguise is a saviour.