This post from the link seems to be about the best explanation:
Exercise is great for lowering glucose because glucose is a rapidly metabolized form of energy, and when you exercise your body needs energy quickly. More often than not exercise will lower glucose, but there are certain circumstances under which blood glucose (BG) can actually increase from exercise.
Blood glucose can rise most commonly if your BG is too high when you start exercising or if you exercise very strenuously. Both of these rises are caused by the same reason, a deficit in necessary insulin to fund the activity.
BG Too High:
BG being too high always means that there is not enough insulin available to bring the BG down to where it should be. Whether T1 or T2, insufficient effective insulin can cause highs and exacerbate highs during exercise.
When you exercise muscles need energy and the quickest form of energy is glucose. Muscles will increase their sensitivity temporarily during exercise so that the same amount of insulin can bring more glucose into muscle cells so that they can work. However, if there is insufficient insulin the muscles will not be able to receive enough glucose from the blood, even if there is more than enough glucose in the blood. As a result, the muscles send a signal that they need more energy, which the body responds to by releasing more glucose. However, since there is still not enough insulin, BG rises and the muscles continue to send the signal for more energy. This is why if your BG is high before exercising, exercise can drive your BG up rather than down, the reason is that there is not enough insulin.
Exercise Strenuously:
The same reasons as above are the reasons why very strenuous exercise causes an increase rather than a decrease in BG. During strenuous exercise the muscles send a signal for more energy, which the body responds to by releasing more glucose. Without sufficient insulin, very hard exercise and sometimes even novel exercise will cause a rise in BG.
At rest, the body uses about 60% of its energy as fat and 40% as glucose. The harder you work, the less fat is used and the more glucose is used until you reach a state of anaerobic activity (weight lifting, fast sprinting) which uses 100% glucose. It is counterintuitive, but the harder you exercise the more insulin your body needs to deal with the increased amount of glucose being released for energy. Often by exercising at a less strenuous pace, can cause BG to decrease with exercise again.
