One non-boogeywoman scenario is that they were hoping the numbers would reverse and look good --so they bet things would get better, but they didn't. It's a chance people will take, sometimes.
If they were willing to cook the numbers a year ago, they would certainly be willing to cook the revision numbers right before an election.
Well a simple (cynical) take is that the current administration is no longer up for re-election, they’ve been replaced by a new candidate and so now it’s okay to blame the current administration.
Equally simple is the observation that the new candidate is also a member of the current administration.
I don’t disagree, but there are also plenty of articles on many news sites which are simultaneously trying to distance Kamala from Biden.
Harris is literally the VP and takes an active role in advising and perhaps even running the current administration. Yes, she's not president, but one would hope the second-in-command would be taking on some responsibility in the administration. Thus, she very much shares this administration's success and failures.