My guess is that Trump votes were way higher than they (Biden) anticipated. As such, they had to get enough ballets in that were not already accounted for in another mail-in ballot or in-person voting. That forced them to pull ballots from counties outside the ones they had prepared for. With limited time and not wanting to run up the down ballot votes in those counties, they opted to select only Biden on those ballots. If too many down ballot races were upended, there would be much more cause within the state to force recounts and audits. If the extra votes cast for Biden don't disturb the down ballot races, there is much less demand from these state and county races to demand recounts and audits.

What would be interesting to know is if we can see a comparison of totals for counties that were called 100% in before the dump and after the dump. For example, maybe 20% more ballots added to those totals for a given county and compare to the totals of ballots with just Joe selected in that county.

I've seen speculation of that as well (on Twatter). If they filled out Joe's only then they could pump out 1k an hour. If they took the time to fill the form out completely it'd be 36 or so an hour.

Possibly that, possibly because they knew, or hoped, the RINO party wouldn't back up Trump and fight them on it as long as the local guy got his.

Yeah, a lot of these states don't have straight-party voting (Pennsylvania in particular), and it was 3 in the morning when they paused things to create and sneak in more ballets. They totally underestimated how many they would need to steal the presidency, thus the 500,000+ "Just Biden" ballots.

I've heard that it's easier to prove local fraud then national ones, so in a way it's a form of cover.