Why would Principal call about a procedure that has nothing to do with why I am receiving LTD benefits?

I am 56 years old. I have been receiving Long Term Disability benefits from The Principal and SSDI for five years. The reason was severe and advanced idiopathic peripheral neuropathy, most like genetic or auto-immune mediated. This diagnosis was backed up by repeated nerve conduction studies and a rural nerve biopsy. I have had several surgeries to correct foot deformity and have had multiple fractures and dislocations of bones in both feet. I have chronic nerve pain and fatigue, and need to rest in bed several times a day. Since going on LTD I have also become diabetic, and have been diagnosed with glaucoma, hearing loss, and severe spinal stenosis. A nerve conduction study indicated that the spinal stenosis was not causing the peripheral neuropathy; the neurologist said there was no reason to ever have another conduction study as my nerves are essentially gone. Last week I had a partial replacement of my left knee. Today I received a call from The Principal inquiring about the procedure and the physical therapy I am receiving. Why would The Principal take an interest in this, as it has nothing to do with why I am receiving LTD benefits? I am very concerned that they are going to somehow declare me able to work once my knee has heeled, as this is the only reason I can think of for monitoring my recovery. Is this possible? Should I be concerned?