George wrote:
....
How about this:

The free-wheeling upper wheel gets ahead of the driven, but loaded lower,
causing the blade to bunch into the gap. Doesn't take much difference in
speed to start the process, which then increases in effect as the bunched
part slows....

The upper wheel isn't free-wheeling, it's driven/pulled by the
blade...the blade makes the wheel move, not the other way round...