The few spots on your redbud are anthracnose, a fungal disease, but that is very minor and common on many species of trees this year because of the cool wet weather.  That is not causing the leaf yellowing, however the yellowing may be serious. 

Many things can cause leaf yellowing and often indicate a root or stem problem.  Look closely at the base for cankers or injury.  Mulch should never be in contact with the trunk and no more than 2-3 inches deep at most. 
We've had so much rain that soggy soil could be a problem.  Redbud does not want to sit in a saturated soil. (Plants need oxygen in the soil for their roots, and overly wet soils crowd out all the oxygen.) 

For other possibly causes, see our website publication, "Common Abiotic Plant Problems": http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG86%20Common%20Abio...  It may have the answer.
If you see any more signs or symptoms, get back to us.
ECN