This happens to me too sometimes when I'm not paying attention. I try to keep the index finger of my left hand up close to the presser foot when I'm coming to the end of a seam. When I remember to keep it close and hold the fabric in position, the seam comes out normal.
I like cathe's idea too. I don't seem to have the problem when I'm chain piecing because I have another piece of fabric right there in position just before the end of the seam. I think that's doing the same thing for me as what cathe suggested to you. :)