My money is actually on Polish special forces (or one of the Baltic states), in an effort to force Germany to be serious about weaning itself off Russian natural gas.
It didn't make much of a difference to Germany since the gas flow via NS1 was already switched off for a while and NS2 never had delivered any gas before the sabotage happened. In the end it was more of a symbolic gesture to freeze the status quo that was already in place anyway.
Russia warns Europe that it will freeze to death if help to Ukraine will continue -> Russia stops NS1 to demonstrate their economical superpower -> EU companies are looking for $18 billion compensation -> NS1 blows up to make an excuse.