Capping the deck is used on the gaming tables, so it might have been a cheating technique before coming to use in conjuring. The idea was published in Ellis Stanyon's Magic, Vol. 13 No. 2, Nov. 1912, p. 12, using a book and a deck. The desired cards were hidden underneath a book, and then the book was dropped onto a tabled packet to add the cards secretly.
Twelve years later, a magician writing under the pen name of “X” suggested using a card case instead of a book. A desired card was hidden beneath the case and was loaded onto the deck when the case was laid over it. See The Magician Monthly, Vol. 20 No. 8, July 1924, p. 95.
Walter B. Gibson later introduced a still more natural, motivated context for the load by using a prediction in an envelope (with the force card hidden beneath the envelope). See his “A Spirit Card Trick” in Two Dozen Effective Practical Card Tricks, 1927, p. 30.