Leaving base too early

[fredhjr]

NFHS Softball--Runners on 1B & 2B, 1 out. When pitcher releases ball to batter, umpire verbalizes "No pitch", runner left early. Offensive team begins to take the field, defensive team begins to leave the field. Confused, I asked a fan what out I missed. He said the umpire called both runners out for leaving early, thus 3 outs. I was unable to verify this in the NFHS Rule Book or Case Book. My thinking was that only one runner could be called out in this situation. By the way, it was a 3 man umpire crew.