There is growing DNA evidence that other species of hominins (close relatives of modern or recent ancestral humans) persisted into the earliest of historical times, at least on Flores Island ("hobbit" fossils have been dated as recently as 4000 years BP).
Sumatra is densely enough forested and sparsely enough populated that it isn't much of a stretch to imagine a species of primate, possibly even a hominid or hominin, living there undiscovered by mainstream science. That part of the world has seen recent "discovery" of large mammal species (similar in body mass to the in-question orang pendek) on Java.
My own expectation is that the pendek is probably a subpecies of orangutan, isolate from the main population on Java for tens of thousands of years, or a closely related species of great ape, but, like the North American sasquatch (under whichever of a dozen or more names), only those who understand the reality of dense forest can truly realize how easy it would be for something to live there into the present without zoologists or anthropologists knowing they were there.
The evolutionary pressure behind such a creature is the same that led to humans arising in a land where apes were present and doing fairly well -- a reproductive advantage provided by some change from what was there before. Assuming they started from orangutan, living in a different environment might have provided impetus to walk bipedally, and oddly enough, swimming routinely is thought by some to be responsible for the "hooded" shape of the human nose, compared to the flatter noses of most great apes.
If, instead, starting from a hominin population, insular dwarfism is a likely cause, as is thought to be the case with Floresiensis -- or perhaps they are Flores man (which might have been spread over multiple other islands the last time the sea was low enough to make that region more of a peninsula than an archipelago). The (modern human) natives of Flores island claim to occasionally see "little people" in their own forest, on an island about a hundredth the size of Formosa. If they did spread during the last ice age, there's no strong reason they couldn't have survived to the present.