Shaking Camera

Hello,

So earlier this year, I got my Iphone 6s plus as a gift and it had a problem two days later sent it back and don’t know if it was repaired or replaced.

but it was brought back, after some few weeks I noticed the Back camera shaking for sometime during a video.

I left it Because I mostly take pictures and I didn’t want to return the gift with another problem to the person.

But recently it’s gotten real bad that I can’t even take a picture.

Now my phone screen is cracked a little bit that was after the camera started shaking.

My Question is what causes the camera to shake?

Because with the cracked screen can that cancel out my warranty if I want to use it.