I am by no means an expert either but this is how I understand it. First half of the equation is of course genetics as others have said but the other half is proper nutrition. From what I know, how an animals rack will form is dependant on the animals over all health and the nutrition it gets in its feed. It seems to start as early as January before they drop last years antlers. If they can find lots of nutritous food then come may when they are growing new antlers they will be healthy and the antlers start growing rapidly. Form then until they finish growing antlers if a drought is on they may have a hard time finding nutritious feed once the summer heat is on and the nutrients will have to go to other body functions. Elk are so big that if they must eat the most nutrient rich plants because they use so much energy just finding food and eating it.

I noticed exactly what noway said he noticed here in Utah and it would make sense. Early in the year when the snow was melting the grasses were green and full of nutrients when the first part of the antlers were growing. After the summer months hit last year it was very dry and green plants were rare and remaining plants lacked the nutrients the elk needed to grow those trophy racks.