I guess at speed there could be a significant enough pressure difference inside the cabin and out to cause this. The air being forced up and over the windshield probably creates an area of low pressure in the center of the roof.
Being pushed upwards could also have something to do with the rubber lined frame of the glass - when the glass breaks - the track the glass sits in is probably pinching it - It is watertight afterall so there is some moderate pressure on the rubber. So perhaps pressure from the seal/track is causing the shards to buckle upwards.
Anyway - just two notions I pulled out of my ass.. I wouldn't assume it's scientific fact but they seem plausable enough.
Being pushed upwards could also have something to do with the rubber lined frame of the glass - when the glass breaks - the track the glass sits in is probably pinching it - It is watertight afterall so there is some moderate pressure on the rubber. So perhaps pressure from the seal/track is causing the shards to buckle upwards.
Anyway - just two notions I pulled out of my ass.. I wouldn't assume it's scientific fact but they seem plausable enough.