the term "swamp out" as it relates to cleaning,

Could be someone just confused a couple of terms as mentioned here. A couple of other words that seem to be mixed up in some contexts are : hodgepodge and higeldy-pigeldy.
Then, on the othr hand, it could just be a person with a carefree relationship with language (and I love those folks) used the phrase and it gradually took on a life of its own and is spreading like soft butter on hot toast.