I agree with you, it generally makes absolutely no sense for Russia to be behind this (at least not the mainstream Russian state, I have my doubts about their internal unity). What's more, I find the timing of this incident odd, having happened just after the victory in the Italian elections of the far-right, who is somewhat ambivalent towards Putin.
The only conceivable motivation I can come up with for Russia would be a 4D-chess false-flag. If they were to already assume that there's no way the EU will give in to the gas blackmail this winter, blowing up the pipeline and sowing doubt on the US or Ukraine itself being behind it could be a way to divide support for Ukraine. However, it would be an extremely risky and expensive bet with no guarantee of it working.