Yeah, that was weird and bizarre beyond belief... Collective gasp in all of Denmark. Still no official cause... I can't decide if its random bad luck, a reaction to something (could be everything from medication, allergy, stress etc... ) or whatever. Fact is, that these kind of top athletes is monitored by docs all the time - on the other hand they might get some steroids for faster injury treatment sometimes so there is that.
Obviously there were immediately some antivaxers that also used this to get some attention...