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Causes would be a health problem to where she cannot hold it or has no control over it or something outside has frightened or hurt her.
In the spring/summer months I find that many dogs have had an encounter with a bee and so any buzzing in the area is enough to deter them from venturing too far out. Buzzing from flies will even have an effect after a run in with a bee. I go through this on a yearly basis with my own dog.
Health wise in this size dog I have seen back issues in which there is little control or something hurts to go out the door, such as going down stairs, or sometimes it is the door itself.
If Ruby is only having accidents of feces both health and fear come into play because when a dog is having a bowel movement they are required to stay in one position longer which puts them in a vulnerable position if it is a fear issue. You may notice some dogs will actually walk and drop stool, and look around them so they feel less vulnerable.
This will also happen if a dog is having pain when they are in that position.