I am sure you are correct in stating that "jumping the string" is an overused excuse. What the percentage is, who knows.
But "jumping the string" can and does happen, though I would say in most instances it is the hunter's fault such as shooting at an alert and tense deer. And the fact that the deer is alert is probably because the hunter drew at the wrong time, or has a noisy arrow rest, etc. and didn't wait for the animal to settle back down before releasing.