One reason could be Jack is a philanthropist and believes in giving back to people who may have contributed to his success.
Another could be Jack expects a return in investment. But how much? Twitter market cap is ~19B. He is giving 1% of Twitter to employees which is 190M. He owns 3% therefore he was worth 570M. To make his money back, Twitter must rise from 19B to 25B. Can 190M invested in its current developers achieve this?
Probably not. Jack is most likely trying to save Twitter. The third option is Twitter is in big trouble and only Jack can see it. This could be a last ditch effort to make short term investments to save the company by investing in the people that matter most. It's a bold move that could pan out in their favor.