possible reasons:
- the de-indexed pages are orphaned (they have no links pointing to them, and no traffic coming in either), so google considers them bad / useless for your visitors
- google is running out of space and removes pages that they don't consider important (silly reason, I know, but it's what I've heard so I'm spreading the "news")
- content on those pages is outdated and, since you've (probably) not updated it, google considers it unuseful to people anymore
- you, or your SEO plugin might have (accidentally) noindex'd those pages and google crawled the site again and saw the noindex tag and... well... this is what has happened
- those pages return 404 errors or other bad / broken / technical stuff so google did away with them
- those pages have malware / phishing or, if they don't have malware / phishing google still thinks that they do
- those pages were performing well and google is getting rid of them to make room for the big dogs as the money spending season approaches so they can give your buying traffic to the big authority sites in your niche (they've been doing this constantly over the last several years, although they used to de-rank pages, not totally remove them, but with this mafia company you never know what they're up to...)
