There is no definite explanation, but mistaking other diseases for Covid is one hypothesis, along with anti-covid rules affecting other diseases.
[1] https://www.santepubliquefrance.fr/media/files/01-maladies-e...
Edit: I don't know why the downvotes (no problem with that), but in case someone thinks it is a conspiracy theory of some kind, Santé publique is an official agency tied to the french ministry of health. The conclusion is of course up to debate but you are unlikely to find better data.