Why did Google do this to my site?

There are a few possibilities.

First, if you browse Google while logged in with a Google account, google will remember which sites you frequent and rank those higher. This is because Google uses 'bounce rate' to determine relevance, meaning if a user clicks a link and then doesn't immediately return to Google, Google assumes that the person found what they were searching for.

If you are logged in and clicking your own site a lot, Google will show you different results than everyone else.

Secondly, Google has multiple servers and data replicates across the vast number of computers in such a way that depending on which bank of servers you hit, you could get different results.

Third, if you aren't doing active backlink building and backlinks aren't accruing naturally, there is a good chance that eventually another site will knock you out of your position.

Fourth, sometimes your site will drop for no apparent reason for a few days and then come roaring back.

And lastly, remember that Google search alogrithm is like an ocean, not a rock. It's in constant motion as it tries to crunch the massive amounts of data spewed at it every second, and also try to factor in the tweaks that Google programmers are trying to put in. Google's programmers aren't geniuses, despite what everyone will try to tell you. Sometimes they put in tweaks that break just about everything. You have to remember that the only strategy is to try for long term stability and that means backlinks, fresh content and relevance.

This is just normal behaviour from Google. You might find your pages flying all over the index for various keywords over the next few months before they finally settle. The most important thing you can do now is accept it, deal with it and keep on building good content and getting good backlinks to your pages. Trust me, you will be happier in a few months if you do..