Forum Moderators: bakedjake
Basically software on a victims (windows) PC that does something it shouldn't. In the case that Brett refers to - your html is intercepted, and altered, adding in popups - which then get passed to IE, which is then interpreted by the Alexa tool bar as a popup. Examples: flyswat, gator.
Other possible causes:
_blank targets - I've had these blocked by the Alexa toolbar.
user requested popups - eg. See full image style javascript code.
I use these on various sites and they get classed as popups in alexa.