SI did not want to leave the AAA. They were forced out due to non-SF student-athletes on their rosters. The WCAL, in large measure, was created in 1967 to accommodate them and, not long after, SH(C), another rather reluctant WCAL convert. That wound up being a huge blow to the AAA which lost two stable, committed, original members of the league. The AAA has been in a steady decline ever since for that and other reasons.
Are you sure of this information?
I think it may have been because of the addition of Archbishop Mitty to the CAL and the number of schools playing basketball.
And then SI probably asked to join the CAL league, as the CAL league was looking for a 12th school to join the league.
Also the Central Coast Section was also being formed in 1965-66.
First C.C.S. Basketball Playoffs were in 1968, which was the first season there was a league called the WCAL.
In 1966 there were 8 schools playing basketball, so there were 14 league basketball games. The CAL usually had 7 or 8 schools playing basketball, with fewer schools playing football
In 1967 there were 9 schools playing basketball, so there were 16 league games.
Archbishop Mitty joined the CAL in 1967.
Adding SI in 1968 meant that there would be 10 schools playing basketball and 18 league games in the CAL.
So the 6 schools (AR, JS, BCP, SF, AM, & SI) on the West side of the Bay formed the basketball league for the WCAL with the schools playing 10 League Games. St. Mary's (Berkeley) joined the WCAL for Football only in the first 2 seasons of the WCAL.
Saint Ignatius made the decision to drop out of the AAA in October, 1966. The 11 CAL principals voted to allow SI to join the CAL by 11-0 vote. This was reported in the newspapers on October 13th, 1966.
So there were 11 schools already in the CAL at this point. The 10th & 11th votes were by two of these 3 schools Salesian, Moreau, or De La Salle. These 3 schools were not in the CAL basketball league in 1967. I am pretty sure all 3 of these schools would have basketball teams playing in the 1968 League Season or the 1969 League Season.
On March 13th, 1967 the West Catholic Athletic League was formed, the 13 principals of the CAL made the decision. The decision was the WCAL was going to still be the CAL with Riordan, St. Ignatius, Serra, St. Francis, Mitty, and Bellarmine being members of the league.
The other league, yet un-named, would have these schools as members: Bishop O'Dowd, St. Joseph's, St. Elizabeth's, St. Mary's (B), Salesian, Moreau Catholic, and De La Salle.
The split became necessary, when it was learned the the 2 divisions of the CAL would have to be placed in different C.I.F. sections - the West Bay schools would play in the C.C.S. and the East Bay schools in the N.C.S.
This was reported in the newspaper on March 14th, 1967.