Strange glass breakage


I have a sliding glass door leading to a small balcony in my new house. The slider is double-paned glass, as is the fixed door it slides over. There is also another fixed double-pane to the left of the door.
I haven't moved in yet, due to multiple problems after purchase, so I am just going over to work, let contractors in, and check up on the place. This afternoon, I went over to the house, and the glass in the fixed door (the one the slider passes over) had a spiderweb cracks in it, throughout the entire pane of glass. I could hear the cracks continuing, and I actually saw a few small ones form while I stood there. The glass is still in place, but it is shattered throughout the whole pane.
It appears that it radiates from a spot next to the frame, about 3/4 of a foot from the bottom. The door was fine last night, as far as I saw, and no one was in the house but a flooring guy, finishing some shoe molding and putting base in other rooms, so there doesn't seem to be any way it was hit. The pane that is cracking is the interior frame, so it apparently can't be from something outside hitting the window.
The temperature has been pretty cold here lately (between 0 and 10 degfrees), but again, the other panes are fine, and it is an interior pane. My only thought was an air leak between the panes, with a temperature differential, but I don't know if that is the case.
Any thoughts?
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Maybe he hit it while carrying tools out. Sure sounds like typical tempered glass that was hit, but it may have been from before and just showed up. It can happen that way. Original hit seems OK, then a day or week later, slamming the door will shatter it.
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That is weird- I had the same thing happen on an exterior pane, but it was summer, so I'm reasonably sure it was a thrown rock from the mower. Good thing it was at an apartment, so management had to pay, not me. If it is a stock size, replacement won't be too painful. Hopefully no strange tints they can't match or anything.
Was the flooring guy there the day you found the breakage, or the day before? Does the balcony have steps to the ground? You sure the flooring guy didn't use that door, like for a shortcut to carry materials and tools? I'm picturing a portable chop saw carried by one end of the base, and the other end raps into the glass. If no steps, maybe he was cutting out on the balcony, to ease cleanup? (Sawdust is a pain to get out of fresh carpet.)
aem sends...
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Mystery solved!
I talked to the flooring guy. He was cutting in that room, and a piece of wood flew off the saw into the window. He said he hoped nothing happened, because he heard some crackling sounds, but they stopped. When I told him about the appearance of the window, he was not completely surprised. I am sure he will make good on it.
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