Collider bias: interview and test scores among new hires

At your firm, a candidate's interview score and coding-test score are essentially uncorrelated across the whole applicant pool. But among the candidates you actually hired, the two are negatively correlated: strong interviewers tend to have weaker test scores and vice versa. A manager worries that good talkers must be bad coders.

Explain why hiring induces this negative correlation even though the two abilities are unrelated in the pool.