Horse walks funny!any ideas what is happening?

my 3 year old light cob has just been sent to be backed and broken.she has been ridden for the first time this week properly with all tack on and i went to watch.she stood lovely and square,was easy to mount and well behaved but i noticed when walking she crossed one leg over the other in a plaiting motion.she didnt do it when led only when mounted!her trot and canter were both straight and flowing and lovely but her walk looked bizarre!as i said she didnt do it from the ground only when being ridden.she is 14.2hh and ridden by a lightweight rider.x