In the absence of a male or female to mate with, a hermaphrodite is capable of self-fertilization
This would suggest that the pressure is a frequent absence of males or females to mate with. This could occur in a species with very low population densities - species living in deserts, high mountains and other places with low levels of plant life to support a food chain. High-level predators (think snow leopards, tigers) need large territories to find enough food (each level of the food chain biomass pyramid is only 10% the mass of the one before - 200 kg of predator needs 2 tons of prey), so they are more likely to experience this difficulty in finding mates than other kinds of animals.
Population densities don't have to be permanently low - a situation (like the famous owls and lemmings in the tundra) with cycles of population spikes and crashes in prey causing similar cycles in predators might be enough to cause sufficiently frequent absences of mating partners.
Population explosion cycles caused by desert rainfall could also provide a situation where it would be beneficial. There's a boom in plant life after the rain, followed by a proliferation of herbivores, which march/fly off to find more food/territory. After the plant life dies off, herbivores start to die off, leaving behind possibly isolated individuals who can't find a mate.
Such species are perhaps unlikely to be social (due to low population density), or they could be social at times of peak population.
They are also unlikely to 'spawn' huge numbers of offspring; instead rearing a small number carefully to adulthood (as most birds and mammals do). If they were capable of spawning thousands of eggs at each mating, the rareness of the matings would be less of a problem. Producing only a few offspring at a time would make it more advantageous to increase 'matings' by allowing self-fertilization.