Here's what the BART website says:
Let us know if you have other documentation.
In part, it may be what the interpretation of "forced to withdraw"
means

[www.bart.gov]

Between 1957 and 1962, engineering plans were developed for a system that would usher in a new era in rapid transit. Electric trains would run on grade-separated right-of-ways, reaching maximum speeds of 75-80 mph, averaging perhaps 45 mph, including station stops. Advanced transit cars, with sophisticated suspensions, braking and propulsion systems, and luxurious interiors, would be strong competition to "King Car " in the Bay Area. Stations would be pleasant, conveniently located, and striking architectural enhancements to their respective on-line communities.

Hundreds of meetings were held in the District communities to encourage local citizen participation in the development of routes and station locations. By midsummer, 1961, the final plan was submitted to the supervisors of the five District counties for approval. San Mateo County Supervisors were cool to the plan. Citing the high costs of a new system-plus adequate existing service from Southern Pacific commuter trains - they voted to withdraw their county from the District in December 1961.

With the District-wide tax base thus weakened by the withdrawal of San Mateo County, Marin County was forced to withdraw in early 1962 because its marginal tax base could not adequately absorb its share of BART's projected cost. Another important factor in Marin's withdrawal was an engineering controversy over the feasibility of carrying trains across the Golden Gate Bridge.