Looks like Mandevilla to me. Also in too small container & got too dry.
Too chilly temps & not enough sun can cause yellowing & leaf drop.
Rose

Yes, it's a Mandevilla, and it really needs to be repotted.

Thanks folks for your help. I had recently repotted it..... How large does it's container need to be? Low temps shouldn't be a factor, it's been 105-110 here in NE Kansas.. I'm using it as a hanging basket on the north side of my house where it gets 6-8 hours of good sun each day.

{{gwi:22047}}

Kind of looks like spider mite damage.

That basket is a bit on the small side - Mandevillas get enormous. Also, like all evergreen plants, they are constantly losing leaves all year round rather than in one big fall. However, there should not be as much yellowing. I think it really is worth replanting it in a bigger pot (and try to use a good loamy mix and not the usual soilless rubbish a lot of stores sell). I suspect watering is also an issue (I no longer do hanging anything after a horrendous summer when I had 17 and was driven insane, dribbling water into dried out baskets too high above my head).
By the way, they are remarkably tough plants - mine recovered from almost complete defoliation after taking it inside to over winter (it would have been better in the greenhouse).