too much water?

Or too little, if it's in a container. Maybe nutrient deficiencies in that case, too, especially magnesium or potash. Possibly also sodium damage, if it's near the sea--we get a lot of that in our salty soil, here.

In my experience it is easier to overwater tarragon than underwater it. Devon is a very wet area so if it is outside...

Hmm...it needs almost as much water as mint, in my experience, but that may just be our low humidity, here.

Mint in our climate has to be kept within a container or it gallops for miles!

Same here, but it usually stops when it gets to the neighbor's gravel yard. : )

lol