Thank you for your question. We've been getting a lot of questions about crabapples dropping leaves or branches drooping. I think our wet spring/summer has contributed to various diseases that affect crabapples. Without seeing pictures of your tree, I would say you may have apple scab. Prairie Fire is supposed to be resistant to this disease, but in wet climates it may show up on your tree. You will notice yellow leaves that turn to brown, and eventually fall off your tree. This link brings you to Extension information with pictures and ways to get rid of this fungus:

Apple scab of apples and crabapples | UMN Extension

There is another disease affecting crabapples caused by a bacteria called fire blight. It causes whole branches to droop and is worse in wet climates. We are seeing quite a few cases from homeowners with either apple scab or fire blight in their crabapples. This link has pictures and describes fire blight:

Fire blight | UMN Extension