"Generally, the author and initial copyright owner of a photograph is the person who 'shoots' or 'takes' the photo."

https://www.copyright.gov/engage/photographers/

Let's say a photographer sets up his camera, aims it, composes it, focuses it, etc. But he decides the light isn't quite right yet, so he goes for lunch. While he is away, his assistant clicks the shutter on a whim, capturing more or less the exact image what the photographer intended to capture. The resulting photograph's copyright belongs to the assistant, not the photographer.

This is in theory, of course. In practice, this would be difficult to prove, since the picture is likely to be on a roll of many other pictures for which the photographer does own the copyright. The assistant would need some definitive evidence that they clicked the shutter on that exact frame if they wanted credit for their work.