My understanding is that the gubmint popped a guy for something, then flipped that guy, threatening to go medieval on charging/sentencing him so that he'd agree to be a CI.
THAT guy was ordered to get Weaver to cut down a shotgun. Weaver did, took it to the feds, they measured it, it wasn't short enough.
Sent him back, Weaver literally pointed to the hacksaw and said do it yourself.
Against his instinct, he did it.
Feds swoop in and threaten similar to para 1 above. Spy on white supremacists or we'll get you.
Weaver won't play ball. Prosecutor threatens to confiscate the family home, which was impossible under the law.
Weaver asked for confirmation that the family home owuldn't get grabbed, they refused, he missed a court date because he was afraid of losing his family home.
Firefight ensued.
That's purely from memory, though.