Asiatic lilies are one of the easiest bulbs to grow with such stunning results in the summer garden. I can't even remember planting these but they seem to survive on neglect and offer up their blooms when the exuberant spring blooms have died down.
My guess is that these were bought in a mixed package of bulbs from Costco a few years ago when this area was all lawn. They were at the front of the flower bed at one time, but as the bed expanded, are now at the back.
The only thing lilies do not like is shade and poorly drained soil, as most bulbs don't. Damp soil may cause the bulbs to rot. After the blooms are spent, remove the flowers so all of their energy doesn't go into producing seeds and into feeding the bulb for next year's blooms. Leave the foliage until it starts to die in a few weeks. If you are so inclined you can always give them a handful of organic fertilizer, but they will do fine without. They are not bothered by disease or insects in my garden, and the bees love them. If their beauty and their ease of growing doesn't win you over, remember too, they make wonderful cut flowers!