Rocket motors on bazookas have to get to high speed rapidly. The rocket coming out of a bazooka tube has to burn in a millisecond so the exhaust doesn't burn the person firing. Some metals are included so they don't burn so fast. Standard type missiles, where they burn for several minutes, are designed to to go on a hundred miles or so. So you mix in something to keep them burning.
Most motors have wires running through it, or to give it structure, and include solder.
Antimony is in solder or the lead could have been part of the wiring. Sanders admits he had no idea what to look for when he began wondering about solid fuel. I spent a lot of time getting up to speed. I called government labs. I called Los Alamos, N.M., several times, and UCLA, the University of Virginia and the University of Illinois.