My gut feeling is that this is related to Trump's illegal attempts to overturn the election
The documents in question may be about the fake electors Trump and his enablers tried to install. It could be about the meddling with the Dept of Justice. Possibly about the pressure campaign on Georgia and other state officials to "find the votes." Or it could be related in some way to Trump's mob that attacked the Capitol on January 6. The big question really is what was in these classified documents that Trump wanted to hide?