Well there are actually fixings every hour but they don't all attract anywhere near as much attention as the big ones (i.e. FFT / ECB / WMR).
Personally I hadn't really noticed extra activity at the end of every hour if I'm honest. Do you have any harder evidence to support this, or is it just anecdotal (not saying you're necessarily wrong, just interested in how you arrived at this theory you understand).
GJ