Yellow card reporting is an imperfect system. It depends on Drs using it and I am sure that not every adverse event post vaccine does get reported if the Dr doesn't think it's relevant even though it's supposed to.
Younger people will definitely be investigated more than older people
if they are ill.
I would think that after the EU publicity U.K. Drs thought 'ooh yeh what about that case I had last week' That's human nature
Also there was publicity in the medical press to help Drs identify and report cases eg the British haematology guidelines I've posted a few times.
So it stands to reason there will be more reports after the first one and does not necessarily mean there was a surge in cases just a surge in reporting of cases
(Like ADHD and autism numbers rising over the years is better recognition not more autism)
This is why the apparent sex and age association could be sampling or reporting bias (if you give more young females the vaccine then you'll get more cases in young females and those cases are more likely to be reported than in elderly men)
Or it could be chance still. These cases might have occurred anyway by chance and we are just focusing a lot on this rare condition and finding more naturally occurring cases now
Or it could be confounding (ie the reaction has a different cause that so happens to be more common in young female health workers top candidate being COVID itself or possibly contraceptive pill)
I am not saying any of these are the reason but they are possible reasons. Many associations in the history of medicine have later been found to be spurious.
I appreciate people want answers but you need to analyse data to get answers. I remain happy to trust the MHRA to do its job.