Regardless of the specific cause in the given incident we know that speed wobble is a resonance with some kind of positive feedback that builds the resonance. (it could also be a negative control input feedback that is time-delayed just the wrong amount).

With it happening that frequently, I'd be changing anything on the bike that could be contributing. Sometimes (usually?) a lot of otherwise unrelated factors have to be just right to make it happen. Starting with tires even because that's the cheapest and easiest, but it would be a good excuse to upgrade the wheels. Handlebar width. Lube the headset. Weight distribution on the bike, tweaking the rider position.

If it's the case that control input is contributing, the recommendation that I've seen and which has worked for me is "stop fighting it". Also relax in the arms and upper body, but it needs to be almost instant.