What causes your SG level to drop?

Games getting bundled

Bundling, price drops, games being removed from the store or going free to play.
Examples:
1
2

karma

Four things:

1. Game you previously gave away got added to Bundle List. Bundle List is managed by just 1 person and solely based on reports by other user. If a game XYZ goes into bundle todeay or 95% in some region sale today it's likely he will take few days (in rare cases of obscure sales even months) before he notices it and adds to bundle list. If for example he adds XYZ into list in a week, all GAs from today up to this time will have their CV reduced.
2. Base Price of game dropped. SG gets game prices from Steam API, Steam API doesn't provide price history, so if you give away 50$ game but in a year it's base price will drop to 20$ your CV will drop as well.
3. Game got removed from Steam - extreme case of point 2. Same principle - SG takes Value from Steam API, but Steam API reports 0$ Value for removed games. Your game got removed from Steam it's Value drops to 0$.
4. You had niflated level due to Steam API bug - Steamk API bugs regarding bundles and packs - if bundle is cheaper than games in it API will report pack price not item price. So you may make GA for let's say Memoria - 20$ Steam Game, but during pack sale it may be worth 100$ or more. after bugs ends it's Value drops as well and your level may drop if it was too high because of a bug.

Off topic: If you want to win some more, you might want to join Unlucky7.

lack of faith

Well, Bioshock went down to 0CV when Remastered came out, but you should have noticed that sooner than just a few hours ago.
I'm not really noticing anything else of significant note in your sent list, however, so it'd likely be down to some minor price drops or a retro-bundle.

You seem to have resolved the matter fairly quickly, however. :)