Ah! I bet it's because we recently wrapped the entire question-and-answer area with a form to allow pressing enter from within text boxes in the question area to submit the question (since we're doing that more in perseus).

Because the whole question area is a form, this button is submitting the form (and thus grading the answer). It's particularly noticeable because this exercise was grading even if the user didn't move anything from its original position.

We should prefer the link styled with simple-button to both avoid this and also so it matches the style of other buttons on the site.