Why does montesquieu believe that disobeying laws leads to a loss of liberty?

Because He  believes that if citizens disobey the law, that liberty would break down.
According to Montesquieu, law is the one that prevents a civilization from doing what most animals in the wild do.
without the law, the strong would have the total freedom to abuse the weak. So, even though law is definitely restrictive in nature, we could argue that it would provide all citizens with equal amount of freedom if it obeyed by all.