It's because the servo can never find it's absolute 0. It sits just above 0 so corrects it self, but then overshoots in the other direction. Then it tries to go back, and over corrects again.
When on load, the load stops it over correcting.
Sanwa used to have this (and label their servos up to tell you!) but they appear to have found a way of stopping it happening with recent new models.
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Matthew White
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