There are several reasons that could be giving you this problem.
Firstly though is the amount of leaf large ?
Cherry and plum will both shed leaf fairly heavily once the fruiting stage has passed so any small amount could be quite natural.
Hot, dry weather will again cause leaf shedding.
If this is the case then next year they should be fine.
It may be wise though to watch for further symptoms
If the yellow leaves are confined to just one or two branches on your tree then the possability of a fungal infection may be the cause.
Keep a close eye on the infected branches and if they defoliate much sooner than the rest of the tree then these should be minitored closely next year.
If the yellow leaves appear next year prior to fruiting then it would suggest that they have a bacterial infection.
This would call for removal of the affected branches and painting the cuts.
All that said....be patient. Plums and cherry are both tough old trees.
I don't know what area you live in but there is one other desease that could affect you maybe.
Look at this site.