When sewing with very high pile fabrics, you will find the pile fibers tend to have a life of their own, getting in the way of aligning and stitching the seam. While the high pile is trimmed from the seam allowance are to eliminate excess and bulk within the finished seam, if the pile is long enough it tends to get stuck within the seam stitch if not kept out of the way in the seam alignment process.
Sewing Tip: To keep high pile flat on the garments right side and prevent it form getting caught in the seam stitch, use masking tape to keep high pile pointing away from the seamline(s).
The masking tape should be applied before the seam edges are aligned and pinned. The addition of the masking tape will also preserve the high pile located along the seam on the garment's right side, protecting it from accidentally being caught and then trimmed from the seam allowance area once the seam is complete.