Only in exceptional circumstances and with great reluctance do we actually delete old questions with lots of votes, for obvious reasons. Mostly, they're just closed or locked. This answers your first question: usually they're deleted for reasons of moderation, not casually. One such circumstance is plagiarism.
All I'll say on this point is that we found another (yes, another) major, major plagiarist and nuked a bunch of posts. You weren't the only one with rep loss, as is expected when on the order of 20 posts vanish.
Unfortunately, I'm not particularly keen on posting a list of these questions, for reasons that should be obvious - but be it well known that plagiarism is the cause of this recent loss of reputation.
So, how do we prevent this from happening in the future? Our community has moderators, yes, but predominantly our community is moderated by you all. In the future, it will be incredibly important for people to watch out for posts that look like plagiarism, and flag them. You're given these tools for important reasons, and they adapt to the needs of each site. As a plea to the community: flag plagiarism.
The sooner we catch plagiarism, the less the net impact on the community. If we let it run free, it'll just suck more for everyone in the future. It's up to you all to help the community avoid these situations. Spot it early. Kill (flag) it quick.
As a postscript, most question deletions (massive plagiarism busts excluded) are automatic and occur in response to minimal activity, closures and such. These shouldn't really affect the average user with any frequency. If you ever spot something and have a question, just ask. We'll be more than happy to answer.
