A little more information would be helpful

6.0 is a great number while 8.6 is in the "doing damage to your vascular system" range so that drop is a good thing

If you are up at 8.6 can I assume you have eaten in the past hour or two before you go for a walk?

If so then you have two different factors working on lowering you blood sugar.

You would have an insulin release by the pancreas in response to your body digesting whatever you ate - that is what is supposed to happen and it will lower your blood sugar by some degree

Then add in a bit of exercise and your muscles start burning blood sugar which will likewise cause it to lower somewhat

If that is what is happening then dropping your blood sugar to 6.0, which is a decent post meal blood sugar level is a good thing

If it keeps falling below 4.0 you have a problem