Might just be from a small injury. Push the hair back at the base of the toenail. Look for any signs of infection, or a darkening of the toenail at the base. Also check the pads, and in between the toes.
This breed is known to be their own personal groomer, and not uncommon for them to to put their whole foot in their mouth. Small injuries draw their attention, and that can lead to chewing the area. Some amount of chewing on the feet, or toenails is fine. I think they all do that.
Excessive chewing on the feet can also be a sign of allergies. But with it being just one toe, I wouldn't think it would be allergies. Excessive licking in one area can also lead to hot spots, or yeast if it's a foot involved.