Marciano retired because he heard of Liston's success.
Marciano retired in 1955. Liston fought 8-rounders in 1955.
Marciano retired in September of 1955, at which point, Liston had all but 13 pro bouts, including a loss to 18-5-1 Marty Marshal who was 180 Lbs and broke Sonny's jaw. The argument that Marciano retired soley for the purpose of avoiding Sonny Liston holds little or no water.