Thanks for writing. I'm really sorry, but I can't say for sure why your mandevilla is turning yellow. Yellow leaves are kind of like the upset stomach of the gardening world --- the one symptom could be caused by any number of problems.
Some possibilities include: Rapid changes in weather conditions (especially temperature), too much fertilizer, not enough soil nutrition, old leaves getting too shaded by new leaves, old leaves aging out and falling off, too much moisture, or disease.
You might bring a few samples of affected leaves to the folks at your local garden center or county extension office for more help; they should be able to give you better information if they can see the leaves for themselves.
---Justin, Senior Garden Editor, BHG.com