You say this and I'm pretty sure I've had this exact experience with my GTP! I never heard of it before and it was very unnerving, like you said lol. I also then concluded that something about handling really stressed her out to the point of doing that, and no idea what goes through their minds when it happens. She suddenly became solid as stone and unmoving, and gripping my hand seemingly harder than she would even grip prey, totally breaking off my circulation. She stopped reacting to stimuli and did not move at all, like her brain shut off! I think I might have ended up running her under cold water or something and she finally snapped back into reality and loosened.
Since then, I learned you can unwrap snakes backwards by the tail first haha