1.) It didn't. In some states (IIRC MI and WI), Trump lost the lead in a couple large dumps because large metropolitan areas, which have a lot of votes and lean heavily dem to begin with, released their counts o f mail in votes (which lean even MORE dem) in one shot.
2.) In other states, like PA and GA, they released series of small counts over a few days as they processed the huge number of mail in ballots.
3.) In AZ, we sorta saw the opposite of #2. AZ slowly processed votes that leaned to Trump after initially processing the heavy Biden ones. So Trump ate into Biden's lead over several days but never had enough.