Some of you are reading too much on weekly surveys which are bound to change slighly from week to week due to sheer brownian motion... as any statistician would tell you. Not to mention most of them come from low quality online-only pollsters (HarrisX, SurveyMonkey, Morning Consult, Zogby etc.) which tend to overestate the weight of younger voters, and infamously distorted the polling average of some recent elections.
This is the 2016 polling average without online pollsters of any sort, for instance.
Well, while I post about the weekly Morning Consult numbers, I'm more interested into long-term developments. And I look at all the polls and just eliminate the crass outliners if there are any (mostly on state polls, less so on national polls).
And here, Biden has been practically on the way down since early June. First quickly (normalisation after the boost from his campaign announcement; the diss he got from Harris, from which he recovered) and since mid July then slowly down since then.
At the same time, the only candidate who was consistently gaining votes was Warren, and at a relatively steady pace, even.
Those are changes over periods of months, not just single days.