Glass broken inside in middle of the night.

   Our neighbors, not those next door but still in the village, experienced a strange phenomena last night and again early this morning. They live in a house that is modern in design, all on one floor; no basement, no attic. The house rambles a bit; you enter through a large sun room which opens into a roomy sitting room which opens into a small kitchen. From the kitchen is a hallway that leads to a Great Room on the right and the hall continues past the Great Room and leads to the Bath and three normal sized bedrooms. From the middle bedroom the hall continues to a pantry and then a utility room and the back door. From the hallway into the Great Room are French Doors. These doors contain of several panes of glass. These doors have been here better than 15 years. The doors open and close easily. The interior temperature of the house is controlled by either air conditioning or a furnace. The temperature range from summer to winter is between 60 and 75 degrees. There are two dehumidifiers that keep the humidity in the house close to normal. Last night, around 3 am a terrific crash of glass brought both owners out of bed with expectations of someone breaking in. A through search of the house revealed only that a pane of glass was broken out of those interior French doors. An inspection of the doors revealed that the other panes of glass were solidly in place and uneffected. Nerves shattered the owners retreated back to bed, the male of the household trying to convince himself the glass breaking was the result of changing temperatures. This morning when the pair got up, half convinced the broken glass was a fluke, they again went to the Great Room to look over the door in broad daylight. Again they inspected each pane of glass for evidence of why one would suddenly break. Everything was firm and fine. The couple had breakfast and as the lady of the house was going to the bathroom she again passed the French Doors and something didn't look right. Going over to the door she noticed the pane above the broken one was full if fine cracks. Considering the fact they had just checked each of these panes less than an hour prior, this was a surprise or maybe shock would be a better word. Running her finger over the crazed pane, the glass fell out of the center but remained around the edges. They now have two panes of glass to replace, But what could possibly cause such an unusual chain of events?  I know this entire area experiences the weird. I doubt that I hear even a fraction of what goes on in the area of strange occurances. These people are very close friends. I saw the result of this particular weirdness as they had not, as yet, replaced the glass. Anyone out there care to hazard a good guess how such a thing can happen?