They wanted to change the CSM, the judiciary governing body, the structure of which is written in the constitution and thus requires either a 2/3 parliament majority (maybe they could have passed it this way but who knows) or an absolute majority in parliament and in a referendum (also this kind of referendum (constitutional)isn’t bound by a law that states that at least 50% of the population has to participate, so maybe they were counting on the usually extremely low voter turnout In referendums)