Hostile reputation decreasing when gaining reputation

Because hostile reputation is considered negative reputation, the reputation bar fills the same way as other, positive reputations. The bar fills as you gain reputation.

This can make it look like your character is becoming more hated, since the red bar is filling. However, it really means they are becoming more friendly. Once the bar fills entirely, you'll still increase in rank as normal.

This is most easily seen with Classic reputations, such as Brood of Nozdormu and Timbermaw Hold.