I agree that I think what you are experiencing is related to your exercise. You can have minimal to practically zero circulating insulin and experience a fairly rapid fall in BGs. This is related to enhanced insulin sensitivity (it's actually a bit misleading to call it this because it's actually more of an 'effectiveness to absorb glucose') for up to 24 and even up to 48hrs after a bout of exercise.
When you exercise, the GLUT4 transporters in your muscles (they are responsible for enabling the transport of most of the glucose out of your blood and into the muscles) ploriferate and mobilise so that they are primed and ready to help transport glucose into the cells. GLUT4's are pretty cool in that they can be mobilised by both insulin and also through muscle contraction.
If you've done a decent bout of exercise (and particularly if you are low carb) it takes very little additional 'exercise' for your BGs to begin falling again, because the GLUT4's are still active. Walking is enough exercise to stimulate glucose uptake again, but not enough to get your heart pumping and illicit a counter glucose release from the liver. This is going to happen if your muscle glycogen stores haven't yet been replenished.
It's good advice to monitor and reduce insulin if required, but also consider how much carb you are likely to have 'burned' and what 'deficit' you are likely to have after exercise. Matching this with a reduced bonus could possibly be the answer.