Jetty does NOT automatically starts the debugger. You most likely have set the MAVEN_OPTS environment variable to include -Xdebug parameters. Check with 'echo $MAVEN_OPTS' and you will see something like:
-Xdebug -Xrunjdwp:transport=dt_socket,address=8787,server=y,suspend=n
You can't run two maven processes which both try to debug on port 8787. So change your global MAVEN_OPTS (in .bash_profile when running on osx) or change your MAVEN_OPTS for your second terminal session:
export MAVEN_OPTS="-Xmx1024m -XX:MaxPermSize=512M"