Why and how have liberals supported the fragmentation of government power?

Fragmentation of government power is the internal form of constitutionalism, which is the practice of limited government through the existence of a constitution. It stems from the liberal belief that "power should be a check on power".
Liberals have supported the fragmentation of power due to their belief in human nature, which means that they fear concentrations of power. Liberals believe that humans are ultimately self interested and egoistic. This means that humans, if in power, use it for their own benefit and potentially at the expense of others. This belief is embedded un a Lord Acton quote "power tends to corrupt, and absolute power corrupts absolutely". Therefore, a constitution is needed to prevent concentrations of power, which would be damaging to citizens.
Liberals have supported the fragmentation of power in many ways. First of all, the separation of power means that no group or individual can dominate the judiciary, the legislative, and the executive. A clear example of this is the USA whose congress is separate from the President and the court system. Bicameralism is another form of fragmentation of power. It allows different chambers of the legislative to check each other. This occurs in the UK, where the House of Commons is responsive to the House of Lords.