suggesting that scrolling is an ingrained user behavior.
UX Mythbusters: The Fold
EffectiveUI
112

Or it could indicate impatience for slow-loading sites. If you go back further than 2002 (which I presume is the beginning of your career?) to a time when ISPs sent you disks to help configure your system and modem with their services, people would still build a lot of Flash and other graphic-heavy sites that took time to load. At the university where I began my career in 1997 we observed this behavior in many people.