There are lots of options, mostly intelligence agencies of various sorts taking matters into their own hands. Maybe a European or American group which doesn't want Europe bought off or a Russia group which wants total victory rather than a peaceful compromise. One could imagine an ecoterrorist group wanting to do this in the confusion of the war but the timing so soon after Russia's mobilization makes it look like the explosions are related so probably not that.

The Libyans from Back to the Future?

GreenWar: a hypothetical sibling of GreenPeace. Of course I don't actually believe this.

However, considering the war and today's sabotage purely from a global warming perspective, it could be net positive in the long run. Accelerating the transition away from fossil fuels.

I wonder if the sheer amount of methane dumped into the atmosphere from the damaged pipelines would be an acceptable cost for the hypothetical GreenWar? I couldn't see it, myself. The pipes weren't empty, and thats a lot of quite bad greenhouse gas now making things worse.

It's a tricky calculation for sure. Depends on the likelihood and duration of those pipelines ever being used again, and the take-up of alternate, renewable energy sources - or just doing without.