Tafsir of Ad-Dhariyat 51:40

(While he was blameworthy)

This means: performing that for which he is to be blamed, namely disbelief and tyranny. The verb form used here indicates the commission of an act that necessitates blame. It is like the triliteral root form used when one commits a strange deed. It has also been said that the form is used to denote attribution or ascription, or that it is causative, though this is as you can see [implied or weak].

That which is to be blamed here—disbelief and tyranny—is what the state of Pharaoh demands. This is a matter that differs according to the one described by it; therefore, one should not entertain the notion of how the accursed one could be described with that which Dhul-Nun (peace be upon him) was described.