Sounds like a classic toughened glass reaction to stress. As said previously, exactly like the older toughened glass car windscreens. I have seen similar fractures in a number of different circumstances, and can say that you should regard yourself as lucky that the glass panel was bordered by the the surround on the PV panel.
My first experience of a toughened glass shatter (other than a windscreen) was 1974 when I was studying Engineering at RMIT. One set of labs that we used had benches with copies of common device data sheets on the timber surface, with 1/4in toughened glass resting on top, and no surround. In the course of laying out metalwork for the chassis of a project, I accidentally dropped an automatic centre punch onto the top from about 10 inches. The centre punch landed on its tip, did its thing, and rebounded a few inches and I caught it in the air on the rebound. At the same time as I caught it, the glass began to crack at the point of impact, and then proceeded to shatter in slow motion, taking about 90 seconds to reach the extremity of the 12x 3 foot bench. The top was cracked but fixed in place until the fractures reached into the last corner of the top, at which point all of the stress in the top was released instantly, and all the fragments for about 10 inches around the edge exploded around the room with a sound like a shot. Despite a major cleanup, we could still find fragments in that lab and in equipment from the lab when I worked for the department 10 years later.
I have also experienced explosive fractures on toughened panels in household sliding doors as a result of stone strike while mowing. In one case of a heavy strike, the panel shattered and collapsed immediately. In another strike by a much lighter stone on a hot day, the panel appeared undamaged immediately afterwards, but shattered and collapsed later in the early evening when the panel was shaded and cooling down, as a combination of expansive stress (heating through the day) the impact, and contractive stress (cooling during the evening).
After these and other experiences, I have considerable concern about the use of toughened glass in many situations.