What makes underglaze run?

Recently I made the pot shown below, painting the design on with underglazes.  After I painted it, I fired it to make sure the design did not smear when I applied the glaze.  Needless to say, the underglaze still smeared (Mostly the black).  Why?  I investigated and found this answer on a ceramics message board:

It doesn't really matter how high you fire your glaze kiln but 

you have to fix the underglazes at a HOTTER temperature to keep them from 

moving under the glaze. So if you bisc to 04 then your glaze firing is 

supposed to be at a lower temperature. That's if you want your 

underglazes to stay put.