When sewing on multiple borders as a strip set you must miter the corners or you can stop them with a border cornerstone. If you don't miter you end up with your strip set extending all the way out to the edge of the quilt instead of each color in the strip turning the corner at a 90 degree angle.
Here is a link on a google search on how to miter borders, lots of tutorials out there.
https://www.google.com/webhp?sourcei...quilt%20border
Edited to add, I just found a tute for your particular situation with lots of pics.
http://www.jaybirdquilts.com/2009/08...-tutorial.html