Last time this was posted someone pointed out that this could be because when someone dies they don't bother to report "they died but they also got neck injurys which would have caused long term damage" so since less people died, the more minor reports could have been made.
Basilar/basal skull fracture is what makes helmets dangerous in general, i.e. landing face-first on motorbikes often led to cracked bones at the bottom of the skull, often fatally. There were reports about increased rates among cyclists when helmets were made mandatory. I consider that pretty serious and not minor. Nothing is without risks it seems.
