Of all the possible accusations he could level against them, why did Joseph accuse his brothers of espionage? Joseph was afraid his brothers would utilize their visit to Egypt to investigate his whereabouts. By accusing them of being spies, he prevented them from asking too many questions. For no one who is accused of espionage is likely to make too many inquiries about a head of state... (Rabbi Avraham of Pshischa)
Reprinted from Parshat Mikeitz 5762/2001 edition of L’Chaim