Gas sales. Bingo!!! locking out and weakening their direct opponent.. their dirty and expensive refracked gas !!!
Because we all remember, not so long ago, Ted Cruz and a bunch of other senators public outcries about the then soon to come completion of NS2. Their speeches reveil a number of potential moralistic and economic motives. Certainly the US has the means to easily pull that stunt to damage important Russian property.
And, it has A REAL INTEREST to have no gasflow in NS1&2.
Also, the current enforced media redderick also raises doubts about the legitimacy of unproven claims of the nature of the perpetrator.