Since there is no direct effect (the pipelines has been out of use for a long time), the effect must be indirect. As such we can't know and will likely never know for sure why it happened or by whom.
It could be Russia. Russia is the prime suspect in 99% of the cases when there is a hostile military operation on Swedish/Danish water. It isn't like Russia were using the pipelines, and it possible that spinning this as an attack on Russia will result in popularity gains for Putin.
People in most of eu are getting angry at their governments and demand some sort of negotiations so the pipes get reopened before people start freezing to death in their homes.
With pipes destroyed, even if people get mad, most governments can now just say "there's nothing we can do".
So, why are the politicians telling people not to heat their houses above 19°C, to wear sweaters at home, to limit showers to a few minutes and why are eg. steel mills already shutting down in germany?
It could be Russia. Russia is the prime suspect in 99% of the cases when there is a hostile military operation on Swedish/Danish water. It isn't like Russia were using the pipelines, and it possible that spinning this as an attack on Russia will result in popularity gains for Putin.