It could possibly be one of two things.
Frangipani, more commonly known as Plumeria, are very susceptible to rust fungi.
This foliar fungus will yellow and eventually drop the leaves. Spraying anti-fungal treatments on your tropical friends every 3-4 weeks will help reduce the amount of rust.
Although they grow in hardiness zones 8a to 11b, cold temperatures in the winter (at or below freezing) may put your Plumeria into dormancy. Requiring no light, soil or water.
According to The Plumeria Society of America, founded in 1979, it is recommended to store these plants in a dry place where temperatures remain above freezing. This may be a closet, garage, attic or spare room.
If your Plumeria is planted outdoors, another good alternative is to cover it with a burlap on cold nights to help prevent frost damage.