This is a time of year when the crops are just beginning to grow - will they keep on growing? Will the pregnant farm animals keep on and give birth to healthy animals? There may even be something symbolic to their crying, since the king has to cry on the 5th day, when the high priest slaps him. If the king doesn't cry then, Marduk will not be happy, and Babylon risks his anger. Maybe the people feel their crying will support whatever crying the king will be able to do. Another possibility might be that they felt, if the king has to cry, he shouldn't be the only one who has to. Since the celebration happens every year, and things usually go well for the city, I suspect that the wailing of the people was more for the symbolic/sympathetic reasons than for the fearful ones. But I only roleplay being in their shoes, and can't say for sure.