Yeah, just looked this up. The impetus seems to have been leftist infighting:

In the 1955 general elections, the ruling coalition won 35 of the 60 seats in the Grand and General Council. Five moderate Socialist members of the council wanted to break the alliance with the Communists who were close to the Soviet Union and did not condemn Soviet actions during the Hungarian Revolution of 1956. They left in April 1957 to form a new party, the Sammarinese Independent Democratic Socialist Party (PSDIS). This left a perfect 30–30 split in the council that paralyzed the government. Given the deadlock, the Captains-Regent avoided convening the council until the mandatory 19 September regency election to choose their replacements.