Nothing to back it up with other than the idea that this could just be people deciding after covid that they like early voting. That this is just a pull ahead of what were traditional same day votes and given the idiocy in our electorate R's think early voting is "weak" and "not secure" so they aren't doing it to the same rate as D's.
Trends before covid as it relates to voting behavior, not their preferences, might be an apples and oranges situation.
Of course there's the Roe effect too to consider so who knows.