Why achievement is unlocked? Achievement is not a lock, door or safe. You don't get anything after unlocking.
I have an assumption that it came from gaming history, word "unlocked" just transferred from secrets to achievements by similarity of their concept. My hypothesis is based on this wiki article. There were
… secrets, which traditionally provided some kind of direct benefit to the player …
Best Answer
Achievement unlocked originated in computer games and spread to other applications such as Foursquare, via gamification, the process of encouraging participation through turning something that's not a game into a game (for example, Stack Exchange gamifies asking and answering questions with rep points and badges). From there, it was used as on sites such as Twitter either as a mocking joke on when achieving something unimportant or funny, or just doing something slightly noteworthy.
Computer games
Wikipedia says:
...
Xbox 360
The wording "achievement unlocked" was used in the Xbox 360 (released November 2005). Here's examples from early 2006: 2nd, 10th, 17th and 21st January and 22nd February. The last is on the website Xbox360Achievements which lists the achievements available in Xbox 360 games.
Foursquare
The location-based, social networking website Foursquare awards people with "unlocked badges" when you check in to places. For example, you get the Local badge for checking into the same place three times in a single week. People often post these achievements to Twitter, or rather, they're automatically posted.
Twitter
You can find "achievement unlocked" on Twitter, sometimes as a hashtag. Some recent examples:
KleverKevin:
ws:
MegaRan: