The back of this card reads: "Wears a mask while pitching, thus the nickname".
In 1910, Walker pitched in the Pacific Coast League for the San Francisco Seals, under the pseudonym Mitchell. Known for the mask-wearing, and for his hostility toward photographers, "Mysterious Mitchell" began to rapidly draw the attention of fans and press. Walker even started refusing to take the mound unless nearby photographers were removed. However, he was quickly identified after a photographer for the San Francisco Chronicle utilized a new telescopic lens to capture his face from afar. It turned out "Mysterious Mitchell" was actually Fred Walker, a player who had signed with the New York Mets, but disappeared after he was accused of assaulting a hotel maid. [1]
So the card should probably instead read: "Assaulted a woman, skipped town, thus the nickname."
"As a freshman in October 1904, Walker suffered a concussion during a practice session when he collided with another player. The injury initially appeared not to be serious, but later that night Walker became 'temporarily deranged'." [2]