I'll take a leap and ask 'What could be embarrassing so as to warrant keeping them hidden so long?' My guess is the records contain a critical assessment of the Secret Service's performance that day and the night before.
You could well be right, or at least correct that what we're talking about are things of that level.
On the other hand, take a look at the zero-response thread I started on "The JKA records: a bit of perspective." The CIA didn't declassify the last of its WORLD WAR ONE records until 2011, and umpteen World War Two records are still classified. What, you or I might ask, could POSSIBLY warrant World War One records continuing to be classified almost 100 years after the end of the war? What makes sense within the walls of a secretive government agency is not necessarily what makes sense outside those walls.