Not that it matters a whole lot in the grand scheme of things. I am definitely not suggesting that. Chakras tend to act like little black holes (magnets) when energy builds up inside them, sucking (pulling) all adjacent flesh unto themselves involuntarily and bending/contorting the spine in the process. These spinal contortions are usually experienced in connection with the heart chakra more than any other. Where ajna is concerned, is occasional wave-like intense involuntary eyebrow furrowing the equivalent of what happens to the spine when the heart chakra gets too charged?
Yes, as the ajna chakra is purifying there can be an involuntary furrowing of the brow. The ajna is being pulled towards the centre of the head.
This can also be used voluntarily, as part of sambhavi mudra practice. It is nothing to worry about. If it becomes too intense, then self-pace your practices downwards.