one possibility I can think of is that perhaps your windscreen was already stressed: do you defrost it with warm water? Has your car ever been shunted (or something else that could bend the hatch slightly)? Could someone have thwacked it with a ladder whilst it was parked, but not hard enough to shatter the windscreen, thus weakening it? Or perhaps some malicious get with a tile cutter?
If it was already stressed or damaged, the combination of a heavy vehicle rumbling closely past at high speed, hitting a pothole as it did it, and possibly the "Bass Mekanik" CD you had playing through the boot-mounted subwoofer could have cause the glass to shatter.
I agree that a siren on it's own is rather unlikely to shatter the windscreen. In fact, if you look at "Sound Pressure Level" competition vehicles, the big boys often mount metal bars across the windscreens. Why is this? The sound pressure inside is enough to pop the windscreen out of the mounting, but I've never heard of a case where one has shattered (and they go for full-spectrum sound, not just low frequency bass).
