Why is C.S. Lewis so concerned with being locked into a wardrobe?

According to an article on the CS Lewis Institute website:

The actual wardrobe that prompted the stories was one made by Lewis’s grandfather and was in the family home in Belfast. Later, it was moved to Lewis’s home at Oxford and now resides at the Wade Center, at Wheaton College. One of C.S. Lewis’s cousins, Claire, remembered occasions when various cousins along with “Jack” (C.S. Lewis) and his brother Warren, would climb into the wardrobe while young Jack would tell them stories he had invented. It is interesting to note that Lewis mentions a few times that “it is foolish to shut oneself into any wardrobe” perhaps because he always kept a crack of light when he told his stories and also because he was warned. When Lewis sent a draft of LWW to friend Owen Barfield, Barfield’s wife Maud was concerned lest children read the story and accidentally lock themselves in a wardrobe. So Lewis added five warnings to LWW. The wardrobe is such a vivid image that one Oxford boy, after reading the book, chopped a hole in the back of the family wardrobe trying to get to Narnia.

Note that the Barfield's daughter Lucy was Lewis's goddaughter, and Lucy Pevensie is her namesake.

This information seems to originate from the book Into the Wardrobe: C.S. Lewis and the Narnia Chronicles by David Downing.