Being Underwater/high pressure atmosphere
Water's dense and easy to glide in, so everything evolves to glide. If you had a very dense atmosphere, you wouldn't need very big wings, and so the evolutionary expense would be minimal.
As you say, arboreal environments are well-suited for the evolution of gliding(as in the flying squirrel), when the ability to leap from tree to tree reliably and quickly is required. Some people(not many, and even fewer scientists) think that birds evolved as tiny arboreal reptiles, and only later grew into big massive dinosaurs like the T-Rex.
Most people, and most scientists think that bird flight evolved as an extension of running. Wings were originally used for stability while running by dinosaurs like Velociraptor, and once the wings became big enough, enabled gliding over short distances, as a way to conserve energy while chasing after prey.
The animals are really just super-duper-tiny
Because of the square-cube law, flying is much easier when you're super-duper tiny like insects. Hence, the animal evolves gliding when it's small, then through evolutionary pressure, becomes super-duper big, and no longer small.
Near natural hot springs
This one is pretty speculative, but if you're living near natural hot springs or a volcano, then there might be a lot more updrafts, allowing gliding to become a very efficient method of locomotion in your specific habitat.
Another very speculative one. Say there's a plant that grows these huge leaves. An enterprising squirrel grabs hold of the leaf, and wham-o, he's soaring through the air using the leaf as a hang-glider. Now, the leaf has a seed attached, so the tree gets to spread its seed far and wide when it has a squirrel to steer it, and the squirrel gets a vehicle to travel wherever. So it's a symbiotic relationship. You could even imagine the tree evolving to become a cute little nursery for baby squirrels, and when they grow up, they take a leaf from the tree, and spread it to a new place, mate with another squirrel, and once the tree is fully grown, the female squirrel gives birth to her babies in the tree, and they raise a happy family together like good monogamous squirrels.
Spiders like the one in Charlotte's web glide using sacks of spider silk. Now all you need is an enterprising spider who thinks to weave in rudders, flaps, and ailerons, and you've got yourself a little 8-legged hang glider right there.