Radio and satellite comms on the periscopes, among other things.
The major countries all have thousands of merchant vessels traveling in every major shipping lane all over the world. My thinking is navies would be worried about these ships being equipped with sensors that analyze satellite signals looking for what could be intended for submarine communication. Eaves droppers wouldn't even need to break the encryption of the signal, they'd only need to be aware of a signal is being broadcast to only a specific region of the the ocean and that bears other attributes associated with military communication.
Broadcasting over a larger area reduces the ability of an adversary to infer location from the signal, but increases the likelihood of intercept. That alone might not be terribly useful, but it can be combined with other bits of information and form a more complete picture. For example, if you know when and where the sub was last docked, fleet movements, regional events that are of military interest, and you have a signal with a message you suspect was intended for them, then that's enough bits of information to be useful.
There are trade-offs. If you're required to minimize the the likelihood of discovery at all costs, then it makes sense to never even attempt communication while underway.
That would require you to be able to decrypt the encrypted signal that the satellite is broadcasting. Which you can't.
All these third party observers see is a bunch of digital noise continuously being broadcast by the satellites. They have no way of telling digital noise destined for a particular sub from the rest of the digital noise. So the signals convey no useful information to them.
https://en.wikipedia.org/wiki/Communication_with_submarines
The article confirms this:
> Salles said the situation now will be toughest for any crews that leave harbor in the weeks ahead, because they’ll know they are leaving loved ones in the midst of the pandemic and, possibly, still living in lockdown. The French government has already extended its stay-home orders once, to April 15, and said it could do so again.
> Salles said he believes those crews will get regular coronavirus updates, but won’t be told of any family deaths until they are returning to the l’Ile Longue submarine base, near Brest in Brittany.