[ZeroGravitas]

The mortality rate went down in absolute terms, but up in per cyclist terms so your theory is the wrong way round. To explain more cyclist injuries you'd have to give a reason why the more committed cyclists that continued to cycle regularly were more likely to hurt themselves.

You could blame the helmets themselves for causing injury and that may be partly true but a more likely reason is 30% less cyclists meaning that car drivers were less used to sharing the road with cyclists. A common bike accident term is "SMIDSY", meaning "Sorry mate, I didn't see you" since they simply didn't expect a cyclist to be there.