Nobody knows how the thumb got there.
One possibility, of course, is that it was originally an entire human, and the crows pecked away everything but the thumb, which they were saving for dessert. Another possibility is that this situation was caused by a bagel. As a species, bagels have survived for thousands of years by developing highly effective defenses against being sliced. Many people try to overcome these defenses by grasping the bagel with one hand and using the other hand to attack the bagel with a sharp knife the size of a canoe paddle. A person could definitely lose a thumb this way, although it's hard to believe that even a really hungry person would simply abandon the thumb and wander off, chewing the bagel.