I think it's basically just the arm springs don't have enough tension to keep the blade firmly on the windshield. Most vehicle's wipers work fine in town and at moderate highway speeds, but when you start running with the the traffic flow (75-80 MPH) they just don't have enough spring pressure. All the wipers on our vehicles seem to do the same thing at these speeds; they wipe well on the up stroke, but poorly or not at all on the down stroke. I've often wondered if cutting about an inch or so off the end of the spring, then re-bending a hook on it would increase the pressure enough to make it work.
