wouldn't be surprised if the investigation by russia concludes some type of remote activation, such as bomb placed physically on the pipeline. while nato concludes some kind of crude missile struck it. cyber attack would be interesting, as both could agree on the method while disagreeing on the actor. but cyber is too cold war and things are meant to be heating up here. i don't see it.
Most likely it was charges placed either by an ROV or divers, and then triggered either by a built in timer, or a remote signal shortly after placing the charges. A remote trigger would probably have to be sound based, so the longer you leave it the higher the chance some fishing boat accidentally sets it off with their sonar, or a passing whale gets the surprise of a lifetime.
However it was done, the depressing fact is it isn't that hard to do, it's just that people usually don't. It's a big, dumb pipe on the seabed, so you really only need three things:
- A boat - easy.
- Some way to get down there - also easy: a diver or an ROV.
- Explosive charge - this is the tricky part, but assuming you can get your hands on the explosives, you could find the specs on the pipeline and try things out beforehand on similar materials. It's just steel covered in a few inches of cement, so you just need to reliably punch a hole in that, and then place several charges to increase your odds of at least one working.