If Bing Webmaster Tools reports a significant increase in 'Pages Not Indexed', what is the most critical factor to investigate first to determine the root cause?

The most critical factor to investigate first is whether the robots.txt file is inadvertently blocking Bingbot from crawling these pages. The robots.txt file is a text file on your web server that tells search engine crawlers which pages or sections of your site they should not access. A sudden and significant increase in 'Pages Not Indexed' often indicates that a new or modified rule in the robots.txt file is preventing Bingbot from accessing a large number of pages. For example, if someone accidentally added 'Disallow: /' to the robots.txt file, it would block Bingbot from crawling the entire website, resulting in a massive increase in 'Pages Not Indexed'. You should carefully review the robots.txt file for any unintended directives that might be causing the blockage. You can use Bing Webmaster Tools to test your robots.txt file and see which URLs are being blocked. Ignoring this initial check and focusing on other potential causes, like crawl errors or sitemap issues, can waste time and resources if the robots.txt file is the primary problem.