I was going to the Camber of Emptiness for the Banittie when in the tall grass before entering the cave ARTICUNO appears (it did flee though)
is this normal?
Yes. If you encounter Articuno 11 more times, it will appear for battle at the Sea Spirit's Den. It won't be easy though; Articuno moves around Kalos, so you'll have to be clever and corner it at a certain location.