When Axel Gembe, a German hacker had compromised Valve, took many developments related files and later gave them to his friends (who subsequently leaked them). While a prototype of Half-Life 2 was the main feature of the leak, many other files and builds of games not related to it were present as well.
The leak was very damaging for Valve back in the day (but not so much nowadays), it also would herald of sorts the many more leaks Valve would face in the coming years.