SURPLUS defined as something that remains above what is used or needed; an amount, quantity, etc. greater than needed. An excess amount.

When prey are vulnerable and abundant, wolves, like other carnivores, kill often and may not completely consume the carcasses, a phenomenon known as “surplus killing” (Kruuk 1972) or “excessive killing” (Carbyn 1983b).
L. David Mech;Luigi Boitani. Wolves: Behavior, Ecology, and Conservation (p. 144). Kindle Edition.

Wolves, as a fact, do not kill for sport; unlike people. Wolves hunt ungulates for food and rarely kill more prey than they can quickly eat. Wolves also partake in caching food, especially in winter. When wolves kill in “surplus” it understood that they are doing so in response to an abnormal situation where prey are unusually easy to take. Whereas normally prey are a challenge and danger to catch, about an 18 to 28 percent success rate. If left alone, the carcasses of the kill also could be a cache that the wolves return to at a later time.

The rate at which wolves kill prey has been measured many times and, as is to be expected, is highly variable. Because both prey size and pack size must influence kill rate, it is useful to express kill rate as biomass per wolf per day. The range runs from 0.5 to 24.8 kg/wolf/day (table 5.5).
L. David Mech;Luigi Boitani. Wolves: Behavior, Ecology, and Conservation (p. 142). Kindle Edition.

Based on study and data over many years, wolf biologists have determined that although greatly varied, the kill rate for wolves show (in general) that they only kill enough to sustain themselves. “Surplus” may be the wrong term to use in general. It may also be a somewhat accurate but unjustified term to use in instances where people are the cause and create the conditions where excessive killing of prey by wolves would be more than inevitable.

In March, wolves killed 19 elk on feedlot grounds near Jackson, Wyoming. The online media and news outlets immediately put all the blame on wolves. The real culprits in this unusual devastation are disease; a bacteria called Fusobacterium Necrophorum (hoof rot), brucellosis pasteurella, Chronic Wasting Disease  and scabies among others that thrive in the unsanitary conditions on elk feedlots where elk are crowded to feed in their urine and feces. The elk are weakened (and sometimes killed) by disease making them prime targets for any predator.

In addition to the cost in tax payer dollars, the feedlots cause great mortality to the elk whereas free roaming elk are much healthier and can exercise more of their natural defenses against predators. As is the case with “spacing” where ungulates stay away from wolf denning areas as well as stay in wolf territory borders. Both of which increase search and travel time for the wolves: an advantage for elk. Disrupting the natural predator-prey order of things by essentially setting up a buffet of concentrated vulnerable prey on an elk feedlot, what would one expect?