Re: What made this happen?

That flippin' scary.

There's a massive patio window in my flat, its on the third floor. When we have high winds, I can see the glass flexing in and back again.

The next time it happens, I'm moving my chair until its over after seeing this.

Re: What made this happen?

This could actually be due to a number of reasons, I may not be correct, though, it is just a guess. Sometimes the binding of the glass in the frame can cause stresses to develop as the glass expands/contracts due to the thermal changes. You could just have the window fixed and place a safety film over it or get a new window installed.