How do trading card booster packs drop?

So today I woke up and noticed new item in inventory which was a booster pack containing 3 cards from L4D 2. From what I see on market these booster packs are new (graph only shows up to 19 of June). No matter the game it seems to always contain 3 cards. There are two actions I can do with it: unpack or sell. There is fancy animation when you unpack the booster ;)

I'm not exactly sure why I got it, I have few guesses:

• I got new badge, power player - this could give me a booster?
• Because of new badge I reached level 10 - maybe every couple levels there are gifts?
• It just dropped randomly for everyone?

Maybe together we could rule out above guesses. It's also interesting from the point of market, to estimate how many boosters are there. With some games they seem to be more valuable than 3 cards separately, but that maybe only initial confusion.

Solution 1:

Booster pack drops are random

They drop randomly for players who have:

• Completed all of the available trading card drops for a game
• Used Steam during the last week

These players will randomly receive booster packs. These packs contain three additional cards for the game, and may contain foils.

Note, this does not mean that you have to have crafted a badge for a game, simply that you have 0 card-drops remaining for that game.

Source: Steam Trading Card Beta Community Announcement

Solution 2:

Per recommendation I extracted this from question to answer.

Valve released an announcement on June 19th on the Steam Trading Group with more details and updated the FAQ. Here's extract from the announcement:

Booster Pack Eligibility - you can now confirm your eligibility for a booster pack drop from your badges page. Just click on "How do I earn card drops", and the dialog will tell you if you are eligible to receive booster packs for that game.

Here's extract from the FAQ as of 29.06.2013 :

How can I get a booster pack?

Once you have received all of your card drops, you become eligible for a booster > pack, which is a set of 3 trading cards that may include both basic and foil cards. Booster packs are granted randomly to eligible users as more badges are crafted by members of the community. Make sure you log in to Steam each week to maintain eligibility.

Once eligible, your Steam Level increases your rate of receiving a booster pack drop:

Level 10: +20% increase in your drop rate

Level 20: +40% increase in your drop rate

Level 30: +60% increase in your drop rate

Level 40: +80% increase in your drop rate

Level 50: +100% increase in your drop rate (i.e. the rate has doubled)

Etc.

How do I know if I can get card drops and booster packs?

From your Badges page, the “How do I earn card drops?” window shows you how many card drops you have earned, how to get more, and if you are currently eligible for a booster pack drop.

Here's how example eligibility info looks like for the Super Meat Boy game (note it maybe differ per game):

I think it answers most questions and doubts.

Solution 3:

A booster pack is randomly distributed to a random user every time someone crafts a badge. Users of a higher level will be more likely to receive one.