In very simple terms, your blood glucose levels are not just influenced by what you consume in terms of food or drink or when. Our bodies have a way to store / create glucose. There are many things that can trigger the release of this glucose - physical activity, stress, illness, etc.
Exercise is one of the bigger factors that can trigger the release of glucose stored in your body. Your liver is quite good at delivering a quick dose of energy when it senses you might need it.
Coming to this late, but just to confirm a previous post: I race that sort of distance fairly regularly, and pretty much every time spike to over 10, with a high similar to yours. The higher the intensity, the higher the spike! Continue beyond half an hour or so, and it comes down just as rapidly (at least it does for me).