I don't believe you said whether or not you measured the border and then ripped. When I rip I always rip longer than the cut border. This avoids a bit of bowing at the end. If however, you have measured and then ripped as stated above, carefully iron and perhaps starch. You need to make sure the one side of the border is not "out of focus" or grain.....If this is the case, then squish it back to the size it should be and work it into the side of the quilt.
I would pin at the end and then ease the possible bunching into the side. Hopefully this shoud take care of it.
I sometime will rip wider than required and then trim off the thread edges.