What led to the creation of the Mulford Act?

To add and challenge some of rrteacher's answer:

Yes, the Mulford Act was specifically designed to weaken and ultimately destroy the Black Panther Party. The NRA was in full support of this bill, which sought to ban California's open carry law.

I would not characterize the Panthers as “militant”. Militant, used in this context, connotes a sense of fear, aggression and power. The Black Panthers, in general, in principle and in nearly every instance, were not initiating violence; they were defending themselves from violence.  When you say, "It was especially aimed at what were called "patrols," where members of the militant Panthers would enter white neighborhoods, fully armed, and walk the streets.” the second half of the sentence is not exactly accurate. They patrolled their own neighborhoods for the purpose of self-defense from the state/police.

"Other times Panthers would follow police patrols, with weapons to protect, as they saw it, African-Americans from police brutality." Yes and no, their goal was to make sure the police followed the law and yes, to hopefully cut down on the very real and horrific violence police had been perpetrating on the African-American community.

The Black Panther Party was a direct threat to the state since their aim was to overturn systems of oppression, e.g. white supremacy, class and capitalism. At the heart of what led to the creation of the Mulford Act were white fear, panic and racism.