i think it was perfectly timed

a) after the primary was informally settled

b) a couple weeks before the candidate was formally sworn in

If that was the case, they would have done it sooner

sooner and there may have been a real primary contest. too risky. they did it with just enough time to sort of "zerg rush" Kamala into the primary without giving anyone time to mount a meaningful attempt at the primary

and unprecedented, move. It’s a huge risk to drop the incumbent in favor of somebody else.

unprecedented, yes. it's the first time in US history since we've been using the primary system that a candidate got the party nomination without a single vote being cast for them

risky, also yes. but they (I think correctly) determined that Biden was a lost cause.

so it was either a) go with the guy you know you're gonna lose or b) go with someone you will probably lose with

b is the logical choice