The only times I have seen straight cracks like these is when there is wall movement or when drywall tape fails due to moisture. This is an exterior wall so you may have moisture seeping in through your siding or from around that window.

It could be that the framing around that window is slack and the wind load is allowing the wall to flex. What is curious is that the cracks are only beneath the window and only vertical. If there was flex, a structural failing, I would presume to see diagonal cracks from the corners of the window.

You can have someone test for moisture content in the wall. I would call someone who does basement waterproofing. They would have the tool.