Tumbleweeds

What is the significance of calling people "Tumbleweeds"?

A tumbleweed is rootless and goes where the wind blows it. Calling a person that could mean he has no firm convictions or maybe no strong family ties.

Merriam-Webster:
tumbleweed -- a plant (as Russian thistle or any of several amaranths) that breaks away from its roots in the autumn and is driven about by the wind as a light rolling mass.

I think it was meant to be ruder than that!

I think you'll find what you are looking for here:
www.urbandictionary.com