two words - stress hormones. I really find this book to be very helpful:
http://www.shericolberg.com/diabetic...s-handbook.asp
once your body starts getting used to the routine, you might find your BGs come down a bit.
Not sure if you're T1 or T2 or T2 on insulin, but I would be weary of giving yourself more insulin before working out - maybe try experimenting with a TINY amount (1/2 a unit, 1 unit?) at a time, but you'll find that exercise makes you extremely sensitive to insulin, so you don't want to go LOW.
Exercise does increase your chances of going low, so always be sure to carry carbs on you. I really like gels like Gu or Cliff - they're tiny, have about 20g of carbs, are rapidly absorbed... something like Gatorade works well too.