All analog functions (triggers and sticks) are linked toghether in the dualshock 4 and can interfer between each other when one of them is not behaving as it should.
A good exemple of that is if you only plug in the motherboard (without the conductive film for L2, R2 and all the others buttons), you can actually activate L2 and R2 with the respective joystick.
An other thing that could have cause the fault on your controller is if you break a solder joint while cleaning the potentiometer or if the conductive film was faulty.
I hope that this can help you or someone else ;)