The glass was sucked in and broke
This occures due to a manufactueres error placing too much desiccant, a drying agent, inside the aluminum spacer. I'm surprised it took 6 years before it manifested.
The Desiccant absorbs moisture, that's its sole purpose in this type of application. However during extrem tempurature changes the excess abbsobent works too good and when there's nothing left to absorb during the flux in temperature it termporaily causes a vacuum resulting in the inside pane imploding towards the outside.
Today there are spacing systems that don't rely on antiquated technology, like Intercept or TPS (Thermal Plastic Spacer), the latter being the better of the two wherein no aluminum is used whatsoever thus the need for a any form of drying agent permantently sealed inside the IG is not necessary.