I'm getting little cracks in my glass with the Trizact belt, what am I doing wrong?

The Trizact belts are an embedded aluminum oxide in rows of tiny pyramid shapes on the surface of the belt. This shape gives friction to the surface of the material you're working and allows the aluminum oxide to be very efficient in its grinding.

One of the drawbacks to this design is that the friction also creates a fair amount of heat on your glass. With the Trizact belts you'll want to reduce the pressure you use on the belt and also remove your glass from the belt more often in order to let it cool. 

The microfractures you're seeing in your glass is caused by overheating the glass at the contact point of the grinding and inducing thermal shock in your glass. Just increase the water flow on the belts and grind for shorter durations with less pressure.