What are ethical reasons for the active pursuit of diversity and, conversely, ethical reasons for a more passive approach? Assume the type of diversity in question is increasing workforce representation of women and ethnic minorities.

Ethically, it is important to value diversity in the workplace. Without actively pursuing diversity, a workplace might exclude certain certain groups and potentially express subconscious biases that prevent diversity from naturally arising. In this way, it is vital to pursue diversity intentionally.
On the other hand, active inclusion could almost be seen as more detrimental simply because you are still singling out minorities and focusing on people's differences, even if the intent is positive. Additionally, it may mean intentionally selecting a minority who has less experience than another candidate, which could be considered detrimental to your workplace and discriminatory against the individual with more experience.
The problem with the diversity issue is that there is no solution that will make everyone happy. Eventually, when equality is common throughout workplaces, it will be simpler to utilize passive inclusion, because there should be no discrepancy in the number of diverse workers.

Workforce diversity is a good thing. The question is how to achieve it, and to what ends. If we believe a company has a moral obligation to help society move towards gender and racial justice, we might think something along the lines of quotas or active recruitment would be the appropriate approach. If we believe a company has no explicit duty to society in this way, a passive approach that emphasizes candidate worker skills and experience would be the logical path forward. Affirmative action, in the broadest sense of the term, is supposed to break apart the stratification and de facto segregation of both society-at-large and local communities and neighborhoods. By making it a matter of company policy to proportionately represent women and racial minorities in the workforce, any disparities in access to education, work experience, or capital that might exist for a marginalized population can be compensated for, enabling upward mobility in communities previously limited by cultural circumstances.
But a major criticism of affirmative action is that it goes against the meritocratic ideals of an industrial society. Qualified white, male, or white male candidates are given an additional barrier to employment because of a statistical assumption they have more 'privileges' and cultural advantages than female and non-white candidates. Every person is born into the world without any say about their gender or race, and each person finds him- or herself working his or her own path through the world. A consistent ethical lens would have us observe the many impoverished and underprivileged white males that statistically-based quotas could easily ignore. Instead, such arguments often go, we should encourage diversity in other venues and in other ways, but should take a less hands-on approach when it comes to hiring policy. That way, the most qualified candidates will rise to the top regardless of their race or gender, and we'll hopefully get closer to a race- and gender-neutral world through the merit of hard work. The right answer seems to lie somewhere in between these two poles. Privately-held companies cannot reasonably be expected to solve the long-standing and deep-rooted injustices of our culture. They also cannot reasonably be completely absolved of responsibility to the culture they depend upon for their existence. Privately-held companies did not create the conditions that led to the under-representation of women in STEM fields, for example. Large tech firms are in a position to address this problem on behalf of society, though, and to benefit from the side-effects that will come from affirming and supporting women's futures in math and science. The ethics of this question are complicated enough, but there's an entirely different discussion to be had about the natural benefits of diversity to any given company and its already existing workforce. Having diversity on paper and by quota doesn't guarantee the maximal effects of true diversity, but it certainly guarantees some kind of beneficial well-roundedness in the workforce that can make the company culture more interesting, resilient, and balanced. These benefits are mostly intangible and unquantifiable. A business can't be completely reduced to numbers, after all.