I'm a fairly novice swimmer, have been doing TI about one year. I've had this problem in backstroke sometimes. For me it happens either when I feel I'm sinking, or if I'm trying too hard to stroke. A natural result of flexing your neck back is that you come up in the water a bit, so this could be a subconscious reaction to water being so near your face. Relaxing my shoulders, getting comfortable with being lower in the water, that has helped with being able to relax my upper body more. Secondly, treating my arm stroke more like a caress and less like a tug of war means I can relax my neck and shoulders more.