Crabapples can certainly be stunning when in bloom. I suspect your crabapple is suffering from a fungus disease called "Apple Scab" which after the leaves are infected for a few weeks, cause them to yellow and fall from the tree. Your Crabapple will need to be sprayed with a fungicide each Spring starting at "green tip" (when you first see signs of very small green leaves emerging from the buds) every week until the cooler, wet Spring weather changes and the fungal spores are no longer actively moving through the air.
I hope this is helpful and good luck.