No clue what causes the HD problems, but I am aware of some HD crashes* posted in the newspapers that seemed to happen in unlikely locations. Maybe...?
I did a tank slap get off years ago on a Yamaha on a very rough snow plow gnawed road. Semi circular chunks of pavement were missing an inch or so deep. I seem to recall reading that front fork geometry might be a significant factor with a bike's susceptibility to this phenomenon. The good news - Speedmasters and Americas forks are apparently of a design that will not readily tank slap. If memory serves, the farther out front the forks ride, the higher the chance of tank slap, but that's only a part of the equation or rake, trail, and all that other good stuff I don't know much about.
Vehicle geometry does weird things. Certain styles of dry bulk tanker trailers and flatbeds when loaded just so will jump up and down at something like twice a second speed at 80,000 lbs. GVW at speeds of around 40 to 45 MPH. Suspension bridges do this too when carrying a traffic jam load. The first time one experiences a half or full mile long bridge jumping up and down can make a body wonder about frequency of recent or future church attendance.
Then there was the outer bridge lanes of Manhattan bridge literally held up on the Greenwich Village side with wooden 12 X 12's lashed together with 3/4" metal cargo bundling strap of 20 plus years ago - another inducement to finding a God along the road. ;-)
*(CT papers often list vehicle make and year, etc.)
