What they found afterwards, on their own without any case brought before them, is that the campaign that preceded the first round of elections (which lasted for one month before the first round two weeks ago now) may have been influenced by outside forces, that a candidate may have flaunted campaign finance laws, and similar matters, and that because of this, the entire electoral process is invalid and annulled. The government has to restart this process from scratch, with anyone who wants to participate registering their candidacy again from scratch.
There is no procedure or standard in any law or in the Construction to specify such a process. The Court invented it from whole cloth, based only on a vague/broad power to oversee the election.