Simple answer - basically, the union in place was controlled by management and scabs. The CAL guys were afraid of getting screwed if they merged with an ALPA carrier and sought to get ALPA back on property.

The CAL guys had a choice of being a scab airline fighting an ALPA carrier in case of merger or allowing the scabs into CAL ALPA. They chose the SCABS. The scabs voted for ALPA and I would assume they were allowed into whatever union CAL had before (their new)ALPA.