Sam and Steve track the Winter Soldier down and he resists their attempts to help him. Violently. During the scuffle, he takes a hard hit to the head.

Now Bucky doesn't remember anything past leaving the World's Fair, 70 years ago. Not the war, or his best friend's science-fueled growth spurt, and definitely not getting turned into a cyborg.

Bonus for him not quite knowing how to control the arm.