Ask Mr. Smarty Plants

Hyacinth bean, Lablab purpureus (syn=Dolichos lablab) is native to Africa.  This is germane to your question only in that the Wildflower Center focuses solely on North American native plants.  However, since your plant problem is general in nature, we may be able to help.

Too much water is a definite possibility.  The growing conditions you describe sound generally favorable for your plant, but if you're getting a lot of rain or are not letting the soil dry out some between waterings, then the roots may be rotting.  If that is the case, you will need to cut the top back hard to save the plant since the remaining roots will not be able to support the abundance of foliage on the vine.

Another possibility is an infestation of thrips and/or spidermites.  Both are tiny creatures which suck feed on plant foliage and cause yellowing and leaf drop.  Check the underside of the leaves with a magnifying glass and look for any barely-visible green or red critters inhabiting the plant.  If spider mites or thrips are there, insecticidal soap will usually take care of the problem.

If you have a good local nursery in your area, you might remove a piece of the vine - not just a single leaf - put the vine in a clear plastic zip-top bag and let a professional horticulturist take a look at it for a more specific diagnosis.