jbeattie wrote:
Frank and Andrew can check me on this, but the reason the bolts back out
has nothing to do with the direction of the crank rotation but
rather has to do with the application of any force to the crank and
the resulting deflection at the crank/BB axle interface -- or a
defective and recalled retaining bolt.

Square taper cranks loosen (less often) for the same reason-- windup of the arm relative to the spindle when cyclically loaded. Nutted square taper spindles were more prone to loosening than bolted ones, because the short and stiff little stud machined onto the spindle end was less able to twist elastically and follow such windup than a bolt of smaller diameter, finer pitch, and longer grip length.

In this case, the wave spline, low modulus material, and insubstantial construction of spindle and arm must allow some relative torsional movement under load, which creates very small but repeated opportunities for the fastener to back off.