What causes a loose shifter?

This is normal with wear and tear of the shifter. The shift knob is actually removeable so it is not uncommon to loosen up over time.

Take a look at the diagrams below. You will see a set screw that will loosen or even fall out and cause this to be loose.

You need to tighten that screw and it should take care of the issue.

Please let me know how you make out with this. Thanks