It’s probably a combination of things, but you are on topic w/ the idea of adrenaline. When you are exerting that much effort (for pulling a 5k in a race or on the erg for example), it’s 99.9% likely that you’re going anaerobic. When this happens, your body is not utilizing the normal glucose-burning properties to make ATP (energy for your cells). So any glucose in your blood stream (and in the interstitial space next to your muscles) is not being utilized. Dump adrenaline and cortisol (both side effects of intense exercise) into your system (which both have a tendency to cause insulin resistance) and it is of course not really helping your glucose remain stable–frustrating to say the least!
In my experience with intense anaerobic exercise like that, I have had to forgo eating a meal beforehand (b/c of the increase in insulin resistance from my nerves, on top of the whole anaerobic aspect, on top of the cortisol dump, etc lol). Or if I did eat, it was something lower in carbs. Also for my endurance activities I’d eat/drink supplemental carbs (like gatorade) but in really intense activities (for me a cyclocross race is my intense/anaerobic activity) I can’t consume any of those carbs…I just drink water instead.
You may be able to help minimize this by increasing basal rates. Remember that any change in your basal should be made 60-90 minutes prior to the activity though (b/c the insulin you’re getting now from your pump isn’t the insulin that’s being used now…it’s the insulin that was delivered roughly an hour ago). This increase is of course dependent on the fact that you will be going anaerobic. If you’re just going to do an endurance workout where you’re pulling at 70% of max effort for 20k meters or something (vs. giving 95-100% effort of high intensity for a 5k or less), then you’d likely not want to do the basal increase b/c that would result in a big crash in your BG level.
Also, when I get done w/ a workout of high intensity, I also give a bolus afterward (b/c sometimes the body does not “realize” that the exercise has ended, and so the liver continues to dump cortisol/adrenaline into the blood stream for a short time afterwards). For example I got done with a bike ride on the trainer last night that was only 1 hr and 15 minutes long, and was pretty high intensity intervals. I ended w/ a BG of 78, but I went ahead and bolused 2 full units (which in a “normal” situation my correction factor is nearly 1:40 so that should have basically put me at a BG of 0 lol). However, my BG still rose to 157 about 20 minutes after I had finished working out (I didn’t give enough of a bolus to prevent that post-ride whip).