You really mean, if Mexico had installed a governor of Texas, who began leaning toward Mexico instead of Canada, Americans forced him from power, and the Mexican government intervened on the side of California Mexicans . . . .
Dunno about the 90%, but the citizens of Baja have often expressed a desire and actually formed movements to join American California. The Mexicans punished them by denying them Mexican statehood for many decades. Then, mot too long ago when they finally let'em in, they broke up Baja, making it two states.
Of course, there are not a hell of a lot of people in Baja, so a real secessionary revolt wasn't really feasible.