Chickens, and spite. That's the official story.[1]
Unofficially, it's indirectly how we got the Civil Rights Act passed as a quid pro quo:
> In retrospect, audio tapes from the Johnson White House, revealed a quid pro quo unrelated to chicken. In January 1964, President Johnson attempted to convince United Auto Workers' president Walter Reuther not to initiate a strike just before the 1964 election and to support the president's civil-rights platform. Reuther, in turn, wanted Johnson to respond to Volkswagen's increased shipments to the United States.
Unofficially, it's indirectly how we got the Civil Rights Act passed as a quid pro quo:
> In retrospect, audio tapes from the Johnson White House, revealed a quid pro quo unrelated to chicken. In January 1964, President Johnson attempted to convince United Auto Workers' president Walter Reuther not to initiate a strike just before the 1964 election and to support the president's civil-rights platform. Reuther, in turn, wanted Johnson to respond to Volkswagen's increased shipments to the United States.
[1] https://en.wikipedia.org/wiki/Chicken_tax