My personal guess. Total shot in the dark,
The US Navy along with Nato navy and potential Nato partner navies have been conducting ops in the Baltic sea.
https://www.newslooks.com/us-navy-assault-ship-engages-in-baltic-sea-training/. There is a ton of data backing up their presence.
What if these ops were used as top cover, warding off other presence, not aware that clandestinely, pmc/merc types pulled off planting the charges. Using the unknowing uniformed Navy as pawns, as their watch dogs. But the mercsvhaving some insight/knowledge of what the military would’ve been doing, they would’ve known how to avoid detection. But this would keep others away so they could work.
That fact that it blew the same day Russian “referendums” ended... I don’t think that was a coincidence.
That is my tinfoil shot in the dark guess.