Our muscle transfer surgery uncovered the cause. In standard lengthenings, several muscles were approached through a single surgical plane. The superficial muscles travel further, by far, to their attachments and therefore travel further than the big deep muscles in any given functional movement. If slow moving large muscles get glued to the longer tendons of the superficial muscles, then big muscles get to move fast too. More fast moving spindles means more spastic.
Spasticity is a second event.
That's what the physiology basic science said all along. We just were not paying attention. First step - acceleration. Second step - reflex reaction to acceleration feedback. First step, velocity crosses a stimulus threshold.