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Potatoes store carbohydrates in their cells as starches. When you cut a potato with a knife, you cut some of those cell walls, but not a high percentage. When you cut the potato repeatedly -- with a blender or a masher -- more cells are ruptured and the starches leak out.

Potato starch is actually sold commercially as a glue. 

In order to avoid this, you have a few choices.
 

First, you can reduce the time in the blender. If the solids are all cooked through, my guess would be that 10 to 30 seconds is all the blending that's needed. You can always taste it and blend more if needed.

Second, you can change your cooking method. If you slice and fry the potatoes first, you can get their starches cooked (and add some tasty Maillard reaction browning) before putting them in the blender with the rest of the soup.

Third, you can try a different kind of potato. Yukon Gold are about halfway between starchy and waxy potatoes -- you might have more luck with Red Bliss (at the waxy end) or Idaho (at the starchy end).