Jared is in control of the kingdom, and has the best access to mount an army. However, the rebellious Esrom and Coriantumr have a loyal following, it is simply more likely to have been smaller than the army that Jared could command. The attack at night is probably our key to understanding how their insurrection succeeded.
Night attacks would be rare and somewhat dangerous. This suggests that we have a body of men who are trying to even the odds by using night against the stronger army. Since they are able to reach the person of Jared, they have attacked Jared’s palace at night, overcoming whatever watch was left on guard. The capture of the king would be equivalent to the capture of the entire kingdom, as witnessed by the captivity of the kings among the Jaredites.