By Tim Reid and Alexandra Ulmer HALF MOON BAY, Calif. (Reuters) -A deadly rampage by a 67-year-old suspect in Northern California was likely an act of workplace violence, authorities said on Tuesday, offering fresh details about the second in back-to-back mass shootings that killed 18 people and horrified the nation. In two separate shootings within three days, 11 people were killed at a Los Angeles-area dance hall and seven others were murdered at two farms in the seaside town of Half Moon Bay near San Francisco. Authorities said they still were trying to determine motives for both shootings….