That pothole IS NOT second impact syndrome.

Second impact syndrome is a potentially deadly condition where the brain has not yet recovered its ability to regulate fluid pressure within the skull after a previous severe concussion where fluid regulation was seriously affected. The prior concussion would have included prolonged serious headaches and other serious symptoms. The new concussion would cause a cascade of fluid pressure disregulation that can cause the brain to herniate into the brain stem or suffer other life threatening conditions.

Your issue sure sounds like neck instability and whiplash. Both whiplash and concussion cause the same or very similar symptoms.

Vestibular disturbances do not necessarily cause a spinning sensation but they can cause dizziness.

Biking a rough trail can easily disturb the crystals, especially with the head tipped forward and to the side. I have had multiple vestibular disturbances over the years, twice this past year. Both were in different parts of the crystal. One time, I was crawling under my motorhome and bumped my head as I was looking to the side. It was an instant dizziness.