As everyone knows, Metal Halide bulbs run hot. This heat can caused thermal expansion in the acrylic. Allegedly, it can also soften the acrylic also making failure more likely. Failure will typically occur as a crack radiating from a corner cut.
It is simple to avoid MH bulb induced failures: don't put a bulb over the acrylic. Put your bulbs over the cutouts. If it means less light in your tank than you wanted, live with it. Bulbs over acrylic are the single greatest cause of acrylic tank failures.