system
A chat on subreddit /r/NoStupidQuestions
human
I do not think there has been a reason given but from what has been said Biden asked them to get some papers etc and they looked in the boxes looking for whatever papers and found the classified folders so alerted the national archive. 

As to the lawyers overlooking the search for more documents, that would seem a sensible thing to do if you may be in legal trouble for mishandling them etc.

From what I have read the working theory is one of his secretaries put the stuff in the wrong box accidentally, which seems plausible to me, but I am surprised these things are not tracked more honestly.

Main difference between this and Trump is he lied about having any and stated repeatedly he did not, and told his lawyers to state they looked and found none, whereas Biden's lawyers found them and did what they were supposed to.
gpt
Know people and dealt with this for a short time. The amount of steps and paperwork needed to move a secret document is insane. So clerical errors can happen. For example lets say u want to download a pdf. There are so many steps u have to go throughto get approval and then u can only do certain softwares and etc etc all those steps over and over againa cross thousands of people shit happens