Booker: "I confess that I have been imperfect. I confess that I've been inadequate to the moment. I've confess that the Democratic Party has made terrible mistakes that gave a lane to this demagogue. I confess we all must look in the mirror and say 'we will do better.'"
There's is no particular vote that he is fillibustering, he is just doing it because he can.
The NYT also theorized that he wanted to do this at some point anyway, because the record for longest continuous speech in the Senate is currently held by Strom Thurmond, while fillibustering the ~~1967~~ 1957 Civil Rights bill, and Booker wanted to top it. He only has an hour or two to go to get there.
NPR also mentioned that there just hadn't been that much going on to filibuster. Nominations, sure, but Congress has been mostly taking a back seat to the presidency, for better or worse given executive actions aren't as permanent as Congressional law.