I wonder what proportion of the web population does this, and if they have a demographic skew. I think I remember starting this back in the late 90's in response to a combination of extremely janky, inconsistent scrolling and tiny monitors which made me easily lose my place while reading a long article. So I'd highlight a section of text before scrolling as a marker, which developed into a compulsion over time. The constant clicking drives my friends and coworkers nuts.