Why "flush out a concept" doesn't hold water

What kind of error is it?

It is a malapropism, which is the use of an incorrect word in place of a word with a similar sound, resulting in a nonsensical, often humorous utterance.

Why do people make this mistake?

The error occurs because "flush out" and "flesh out" sound somewhat similar, leading people to confuse the two phrases. While "flush out" is a legitimate phrase meaning to force something out (e.g. "The birds were flushed out of the tree"), it is incorrectly used when the intended meaning is to develop an idea or concept more fully.

What is correct?

The correct phrase is "flesh out," which means to add detail, substance, or depth to an idea or concept, as if adding flesh to bones to make it complete.