Self Employed for Previous Employer

I'm Self Employed Web Developer and doing business with different people. 

In Turbotax Online I had a question, if any of my self employment income was done for former employer. 

I've replied "Yes", and typed the amount. My tax due increased because of that for about $300. 

 

My question is, I couldn't find any article that says if you work for former employer as self employed, your tax rate is higher? Does anyone knows why tax rate increases if you do project for former employer?