Our family JUST went through this within the past month. Our 8 year old had an emergency appendectomy before Christmas. Her surgeon, after looking at the pathology report determined that a viral infection (She had H1N1, but treated with Tamiflu, not severe) within the prior 2 months can lead to the internal swelling of the lymph nodes, which put pressure on the appendix, pinching it off, allowing it to become infected over a period of time. Truly, this was the first I had ever heard of an explanation - we were in one of the top hospitals in the country.
The virus being an indirect cause. I don't know what lymph nodes are there but that's an interesting thought.