pH of bleach solution slowly decreasing?
Looks like your bleach bleaches your indicator, just like it does pretty much everything else. After all, we don't call it "bleach" for nothing.

Sure, pH may be in fact slowly decreasing, and the bleach would eventually decompose and lose its properties, but these processes do not typically occur in a matter of minutes.

As to the mechanism, it is complicated, as are indicators themselves (usually, large organic molecules). One might think of a huge conjugated system of double bonds that produces color; all of a sudden, an oxidant appears and starts randomly snapping bonds in half. Soon the conjugated system is gone, and so is the color.