Long Term Capital Gain and AGI

I'm having problem understanding how Turbo Tax treat  Long Term Capital Gain(LCTG) with respect to AGI.

My understanding per Tax Code, Long Term Capital Gain is tax separately.  Majority of people would pay 12% flat on LTCG.

So I have scenario I'm trying validate using Turbo Tax Premier.  My current 1040 form show:

  • AGI: 142K
  • Expect Tax Due: 4K

Then I entered LTCG of 10K

  • AGI: 152K
  • Expect Tax Due: 12.244K

How is this possible?  10K LCTG gain has $8,244 tax liability and push up AGI?