If it's the only area I think you're talking about, it looks like it could be breakage oppose to thinning. You know if it looks different. If it's breakage it could be how you tie your scarf, bonnet, it could be from using edge control, mechanical breakage from brushing, etc.
My daughter is experiencing breakage along her entire hairline and I suspect it's because of how she was tying her scarf for years so I switched her to a bonnet. Her edges are also tender to the touch which could indicate clogged follicles so I'm taking measure to make sure her scalp is clear of buildup. It could also be her ponytails/buns so we make sure they are loose.
You're going to have to really assess what the culprit coykd be and stop doing it. Start the process of elimination. Is the area shorter or just thinner?