Poland is a major logistical hub for everything going towards Ukraine. Thus targeting basic infrastructure like energy grid or railroad have to be expected.

On the bright side, using these weapon grade malware is burning exploits and also showing current state and techniques of Russian cyberwarfare which defender can learn a lot from.

Russia is at war with Europe.

The most obvious answer is Russia(or one of their allies like China or Iran) did it because Poland is supporting Ukraine in the war (directly, and also indirectly by letting stuff from other countries be staged and move through Poland).

Russia is currently focused at striking Ukrainian energy assets. Ukraine get energy imports from EU through Hungary and Poland. Hampering energy supply from Poland would but a huge strain on the already struggling Ukrainian network.