Thank you for your question and photo for Extension Master Gardeners. Your crabapple is suffering with a lot of bark loss. There could be several explanations for what's affecting your crabapple. One is fire blight, a bacterial disease which affects the entire tree including the trunk in its late stage. Do you know the variety of crabapple you have? Many are resistant to fire blight. Did you notice any issues with curling and spotting on the leaves, fruit and branches this last season? Unfortunately, there is no cure for fire blight in its late stages. Here is more information from Extension on fire blight:

Fire blight | UMN Extension

If you haven't noticed leaf and branch damage, then I suspect it's sunscald, a winter injury common on small trees, including crabapples. Since you mention southwest exposure, this is likely your problem. For this, you need to wrap or apply a plastic tree guard to the trunk in the fall. You can also repair sunscald to hopefully save your tree. This Extension article has a section on sunscald with all the details:

Protecting trees and shrubs in winter | UMN Extension