Why does glass spontaneously shatter?

We had double-glazed sliding patio doors installed about 18 months ago. On separate occasions over the past couple of months, two of them have spontaneously shattered from around the centre of the pane. Both of them inside panes which rules out unknown impacts from outside.

The first one was in the bedroom; I had just gone to bed and could hear what I thought was rain or sleet against the window. I thought that odd as it was a perfectly clear sky just before I went to bed. I opened the blind and the whole inside window had shattered. It was still in-situ (thankfully!).
The installer is currently in the process of trying to get this one replaced under warranty from the manufacturers.
But when the 2nd window shattered he said he’d have tremendous trouble trying to convince the manufacturer of two separate faults.

He went on to say that if it was a manufacturing or installation fault, then the glass cracks/shatters from around the edge of the glass. But these are near enough bang in the middle (excuse the pun!). At no point have they been hit with anything. Indeed, the installer said that these units are incredibly strong, and that they have trouble breaking them in the skip, especially when trying to break them from the middle on the pane.

I read online about this and how that it could be caused by Nickel Sulfide contaminants during the manufacturing process; these contaminants expand with variations in temperature and it’s the expansion that causes the glass to shatter. That explanation certainly fits with both of my scenarios; the 1st occurrence was at the end of summer; it was a very hot day, but the temperature plummeted in the evening and it was the first night that we had a frost. The 2nd occurrence happened recently (in the middle of winter here in the UK); it had been about -3c for a couple of days and the wood burner had been on almost all the time. That window shattered over-night.

The glazier that installed them says he has never heard of such things happening, but another glazier I spoke to said that it’s not uncommon, especially with large variations in temperatures.

Has this happened to you? Do you know what caused it?