Shaking could be the power supply related.
The 5volt supply should be able to deliver at least 1Amp per servo.
Do you wait until the servo has reached it's position before reversing.
What happens if you pause longer.
4096 is the number of steps of the 12-bit output of the PCA9685.
The code sets a number of them HIGH and a number of them LOW, to generate the servo signals.
Leo..