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.