Supposedly, "the name was christened by the clergy who accompanied the first Spanish explorers to South America. They saw the fruit’s brilliant flower as a symbol of Christ’s crucifixion, sometimes referred to as The Passion of the Cross. The beautiful corona threads of the Passion Flower were seen as the crown of thorns, the five stamens were wounds, the five petals and five sepals represented the ten apostles (excluding Judas and Peter) and the three stigmas symbolized the nails of the cross."
Silence Evermore has it right, there is a legend told about it. Found a wild passion fruit plant in the woods near my home when I was a teen and took the flower, fruit & leaves to a lady I knew who told me about it. It is so unique her neighbor, who was a china painter took it and painted a plate using it as a model.