NSAs don't do near as many Purchase signings as refi's. Local buyers go to a local bank--no NSA needed--closed by local TC, so a busy Buyer's real estate market may translate into refi's a few years down the road--unless interest rates go up.
There have been too many NSAs in some areas for years--which causes fees to go down as there's always somebody willing to do it 'for the experience'. There is really no way to generalize this job across the nation as the local variables are just too area-specific on all fronts. It's a numbers game. A large population will create both more loans and more NSAs fighting for the work.
As to cause--too many notaries willing to work for less than a livable income...newbs come & go when they realize it's not as lucrative as they thought. R.E. Sales isn't even a factor, nor is the number of lenders. Low interest rates are the driving force.