So what's the story behind all these flash posts that show up on searches, that have 100 or 200 seemingly random tags? And the flash turns out to be some kind of game. Is this spam?

Just a few examples:

http://e621.net/post/show/105450/acid-ai-air-angel-apple-ash-atheism-blood-bomb-boo

http://e621.net/post/show/105430/air-airplane-alcohol-alcoholic-ash-assassin-avian-

http://e621.net/post/show/63506/-avian-bird-blush-bones-buff-cats-chicken-chinese-

The tags are there to list what is in a post. If there are over 200 things a post can be tagged with, then it is valid to tag it with those things.