Why do dachshunds do this?

Dachshunds do that because they have no undercoats to keep them warm. Any dog that gets too warm under the covers is bright enough to crawl out or at least stick its nose out from under the covers. As long as you don't roll on the dog, it is very unlikely to suffocate. Yes, dogs have a higher body temperature than humans. We are at or around 98.6 and dogs are usually about 101 degrees.