Sodium lauryl sulfate is a soap that may clean away a protective layer of oils on the soft tissues of your mouth. Or people may be allergic to SLS. Whatever the cause, I pesonally get far fewer canker sores if I avoid SLS and occasionally gargle/rinse with a probiotic solution, such as powdered healthy trinity. I just found out that Rembrandt Natural is SLS free by the way.
Don't know about Rembrandt, but Biotene is SLS free, and easily available around where I live.