If it was just self-interest from those homeowners, they would welcome densification because that would make land values skyrocket. You can see some of that already in areas under the recent round of city zoning changes, where housing prices shot up significantly in potentially upzonable areas the second the new law was passed, even with zero actual practical changes so far.

To me it feels much more like just a significant cadre who resist any change, of any kind, for any reason, who can ignore the personal side effects because of Prop 13 and because their family bought a house in the 80s and they don't give a shit about anyone else who wants or has to live in the city.