Kalani is almost 10 months old. He only does it when I go out the front door and don't come back in right away. I watched him and what he did was jump up at the windowsill trying to see me and when the blinds bumped him he snapped at them and then started chewing the edges
The x-pen is high enough to block the bottoms of the blinds so I can still open the windows but low enough that I can easily step over it when I go in and out.