A couple details I haven't seen posted regarding the D5 launch. Boomerfag here.
First, i agree with submathfag (from notables). The first issue is a depth restriction, which he touched on. I don't think those parameters can be met so close to shore.
The second, I don't think I can say much about. But suffice it to say, the sub has to be at rest to launch. This eliminates the possibility of a purely external hack/launch.
Third, what payload each boomer has, and which boat is the designated alert asset would, as far as I can tell, require a intelligence breach of its own. So even if the weapons system was comped, they would need some highly classified human intel in addition.
In conclusion, I think it's a cabal/rogue sub OR a torpedo launched weapon NOT a D5.
Another possibility is a D5 launch from a different platform. Wasn't there some info about a ballistic missile launcher that could fit in a cargo container? If so, this might be another possible origin of the launch.