When Lon Nol declared his country a republic in January 1970 and removed Sihanouk, everything changed.
There are three dimensions to what happened next: the rapid growth of the opposition to Lon Nol throughout the country; the corruption and ineptitude of the Lon Nol regime; the intense B-52 bombing campaign of the US air force.
United Front (FUNK)
When Sihanouk heard of the coup he immediately threw in his lot with everyone who wanted to resist the new regime, and formed a broad united front - the National United Front of Kampuchea (FUNK). This included the communists and was recognised by them and their Vietnamese comrades as a way to extend their influence.