Almost certainly suspension setup, or maybe tyre pressure. Sounds like you're running out of travel as the bike squats and the tyre is shearing. Andy's right, bobbling is rarely a sign that the tyre is working correctly, more usually that it's sliding around.
Check that you're using all travel available on the rear, if not then back off on preload/compression damping, if you are then add in a little compression damping and check your sags.
If you're not using tyre warmers there's always a chance that the outside of the tyre is getting very hot while the carcass is cool, I get this on the DD bike, but the slides tend to be progressive rather than on/off.
Good luck.
Ali
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