Re: Achilles tendon
Yes, fatigue is a factor as you noted.
If a muscle doesn't have the ability to absorb force as well, it will "jerk" down.
It's the same thing you see with tendonitis -- the muscle has poor input because of the pain -- so the motion is usually jerky on the eccentric which accentuates the forces placed on the tendon.
Hence, why tendonitis can often get worse very quickly.
It's a combination of factors coming together. High reps is definitely a factor too.