First, there is no process or even mention in any Romanian law about annuling an election. The Constitutional Court of Romania has two specified roles in an election: it validates candidates (or rejects them, as it did with another far right candidate in this very election), and it validates the election results (or invalidates them if there is a significant suspicion of fraud in the vote counts, after a recount). They issued a recount order for this same election as well, and then decided based on the results that the first round was valid. If they had decided it wasn't valid, there are laws for when it would have been repeated. However, they later came back to this decision, and quoting a vague article in the Constitution that says that they assure the electoral procedures are followed, they invented this concept of annuling the entire electoral process, from the very beginning (so even the candidate registrations have to be re-done).
Secondly, they did this based on evidence that was public knowledge for two weeks, including the last few times they met and validated the counts.
Thirdly, the evidence in question is vague accusations of Russian interference with no specifics. There are no names, no identified groups, no sums of money except for one lump payment by a businessman. The only clear accusation is campaign finance violations (which the authorities had already blatantly ignored, as the candidate in question had registered his campaign costing 0 RON, which was known to the relevant authorities since last month and to the public for two weeks).