The motivation from the previous nationalist PiS government in Poland was mainly anything they could to do against Russia or against Germany.
So by supporting Ukraine with military equipment, they were acting against Russia. But at the same time slowed down German support for Ukraine, by blocking maintenance and factories for Ukrainian equipment in Poland. Leading to long transports across to other countries e.g. Latvia for maintenance.
Now, with the grain, this has no Russian involvement and their own farmers are unhappy. So there is no way to go against Russia or against Germany. Also there is a new government in Poland, so I guess we have to see how that develops now.
It’s probably meant as distinction from other sectors with Methane emissions, e.g. agriculture, transport, chemical industry…