His explanation, and he, were bogus all along.
Louisiana Sen. Bill Cassidy drew an avalanche of criticism concerning his vote to proceed with the Senate trial attendant to (another dubious) impeachment of GOP former Pres. Donald Trump, Cassidy took to social media to attempt a defense of it. He had the raw material to do a credible job; while the case that trying a former officer of the U.S. has merit through a reading of the Constitution and past argumentation during impeachment trials, the case against is stronger. Still, he could have offered a principled defense of his decision that he used reasoned judgment to make the call.
Instead, he mostly rehashed his initial explanation. He said the Democrats who comprised the majority in the House of Representatives managing the trial – the prosecutors, in essence – spoke to an learned case for permissibility of late impeachment, although briefly adding that the sources for this came from Constitutional scholars, including some allied with the Federalist Society which is an organization that promotes interpreting the Constitution through original source materials supplemented by learned extrapolation, often favored by conservatives. By contrast, he said the defense lawyers provided little of the same, which clinched his affirmative vote.