I have a much older +4 and experienced the same after crossing a particularly rough rail road track. The wobble was so violent, I thought I had a front wheel coming off. That was 5+ years and many miles ago. It has never happened again and I have no apparent damage. It was as though the shock absorber was overwhelmed by the violence of the impact and stopped working momentarily.

67 +4, 5 sp M3W, 3.7 Roadster(s), CX Plus 4