Skin on black bean soup
Whenever beans are cooked, they release a good bit of starch into the cooking liquid. 

The longer you cook the beans, the more starch they release. Boiling, breaking or otherwise roughing up the beans will release more starch. 

Also, canned beans release a good bit of their starch into the water in the can. 

If you have more starch, you'll probably get more of a skin. 

Puddings (also liquid + starch) form a skin the same way your soup does.