Obviously, no one knows. Lots of differing opinions, speculation.
JMO--I tend to agree with those who say he's an injustice collector and a narcissist. He's smarter, better, and superior to others. His needs trump all. From that perspective, I could see him being "determined" that the LAPD will not easily get truck and contents. I tend to think that truck was a hoard of food, uniforms, plans/maps, communication devices,as well as weapons. I think he wasn't thinking too tactically when he torched it, he was furious at the change in his master plan when the axle broke.
At that point it was grab whatever he could stash in a backpack, and then attempt to destroy any additional evidence.
Just some possibilities, and I realize my speculations could be totally wrong.