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

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.