The most accurate explanation for why some people consider 1918 to be the starting date of the Russian Civil War is:
Lenin decided to disband the Constituent Assembly in 1918, which angered people because the Bolsheviks were becoming more controlling.
This event marked a significant point of contention and fueled opposition against the Bolshevik government, contributing to the civil conflict between the Red Army (Bolsheviks) and various anti-Bolshevik factions, including the White Army.