What was wrong with Old Flapjack in chapter 13?

Old Flapjack is the community mule. Elijah prefers to ride him rather than the horse Jingle Boy as it takes much less of a toll on his insides, and also because it's not such a long way down you fall from him. Old Flapjack also comes in pretty handy when it's time to go fetch the mail. Although the Buxton settlement has its own post office, every once in a while the mail doesn't come through for two or three weeks. When this happens, someone has to go to Chatham to find out why.
On this particular occasion it's Elijah who volunteers to go. Riding off to Chatham is one of his favorite chores, but only if he's allowed to take Old Flapjack with him instead of one of the saddle horses. As the boy and his mule approach the woods outside Chatham, Old Flapjack starts acting strangely. He's obviously seen something in the woods that frightens him, and he quickly becomes agitated, snorting and raising his front hooves so high in the air that Elijah falls to the ground. Elijah is then astonished to see Old Flapjack do something he's never done before: run. The poor old mule really must be scared.