Who was behind getting rid of Biden?

Sifry's analysis supports what AOC said. He references Schlozman and Rosenfeld's book The Hallow Parties, where 'the blob' has a powerful influence. It consists of "the amorphous array of donors, elected officials, interest groups, consultants, media commentators, and local power brokers that identify as part of the Democratic ecosystem."

It was the blob that pushed Party leaders to do it. Withholding huge donations was motivation enough to scare them and vulnerable representatives, themselves led by Pelosi. But that still leaves the unanswered question of why the donors dried up and wanted Biden gone. The article doesn't have an answer for it so I will provide informed speculation. 

Biden was their candidate in 2020 because he was perceived as friendly to big business and he wasn't then much of a progressive. Sanders' campaign frightened them with his strong progressive message and in the early stages of the primary seemed he was going to get the nomination. In came the blob that likely also threatened to dry up the big bucks so Pelosi and Obama went to work manipulating the other candidates, including Harris, to rapidly turn on Sanders with promises of political payoff. 

Unfortunately for the blob, Biden took to heart Bernie's progressive, for we the people agenda and got busy implementing quite a bit of it. Part of that was making the blob pay its fair share of taxes which really pissed them off. And all of those hideous environmental regulations and green energy initiatives replacing oil? More pissed. So when Biden's age did start to show, especially in that disastrous debate, they quickly capitalized and used their money as leverage.

It also accounts for AOC's report that they didn't want Harris either. She already established one of the most progressive records in the Senate, which was likely one reason Biden chose her to second chair. It was obvious that she not only supported that progressive agenda but likely had a lot of influence in generating it. Hence Pelosi and even Obama, captured by the blob, tried to push for an open primary at the convention to get her out too.

Little did they suspect though that the trauma of Biden stepping down and endorsing Harris, and the relief and excitement it created, would take off so quickly that she garnered such huge support, including pledges from enough delegates to clinch the nomination. The blob's plan backfired and now we'll have Harris running the show with a continuation of a more progressive agenda, which is exactly what we the people want and the blob does not.