Just as museums add to collections for a variety of reasons, museums remove objects from collections for a range of reasons. If an artwork is considered to be a forgery, in bad shape, or a duplicate of something else in the collection, it may be sold or given to an institution that can learn from such a work.
Also, if a museum changes its mission or goals, it may need to decide if certain artworks in its collection still serve the same purpose they were acquired to.
Dewing's Lady with Lute was deaccessioned from the SCMA in the 1940s, along with nearly 100 other works of art. This painting was sold by Gimbels in 1946, on behalf of the SCMA. In May 1947, Dr. Walter Timme bought the painting, which he later donated to the National Gallery of Art, in Washington, D.C.
It's important to be methodical and reasoned when removing objects from a collection, after which they will probably be difficult (or impossible) to get back. In order to avoid regret, the museum needs to be aware of changing circumstances around issues of taste and space. The importance of a central mission behind a museum's collecting practices is intended to safeguard against changing trends. Additionally, limitations of space can change over time, so this also should not be a primary motivation for what a museum chooses to keep or dispose of.