IIRC, along about the fall of 2013 Hornady released a letter to the effect that due to high demand outstripping supply/production(the result of the panic buying caused by the Obama gun ban proposals in the wake of Sandy Hook) only the more popular bullet weights/types in each caliber would remain in production along with additional production lines/shifts opened up in the attempt to meet demand until the situation stabilized.
The letter also announced the updating of Hornady's product line that would, in light of the situation, be put into motion earlier than planned and certain traditional/less popular offerings would be discontinued in favor of more modern designs.
I seem to remember there was a list including the bullets that would remain in production and those that would be discontinued altogether or limited to seasonal production runs, but I do not remember whete the .312 bthp fit into the grand scheme of things.
It had actually disappeared off radar, at least temporarily, about a year and a half before the Hornady letter came out.
The 6.5mm 160gr RN was one bullet that was discontinued, but due to demand it was reintroduced several years later.
Also, the Hornady .312 bthp was not a Grafs exclusive, I know that Midway, Natchez and other distributors sold it at one time or another. If fact the Midway web page still has a listing for it but it's out of stock and deemed discontinued.