Q:

Why do people tap the back of cameras during filming?

At 58 min 49s, the video above shows on-set footage of a film crew. (Link is setup to start playing from there.)
For a few seconds, it shows people following the camera and repeatedly tapping the back of the camera.
Why are they doing this? Is this still a thing with modern equipment?

A:

That's a method used to add camera shake (as @Catija suggested). The anti-shake features present on the cameras would otherwise make the effect have to be done in post-production, which is far more expensive.

