Resubmitted: what could have happened in the tooth extraction process that resulted in my need for a sinus lift?

Nature vs. surgery: It is more related to the low lying sinus than anything as a consequence of the extraction. With a natural tooth, the sinus was already low and surrounding the roots of the existing tooth. However, with the tooth out, the placement of an implant has different considerations and is a surgical procedure requiring sufficient bone for its placement and long term success.

Bone thickness: To build on the previous answer, if the natural bony wall separating the sinus and mouth is too thin, it can't support an implant properly. A sinus lift procedure thickens this bone in preparation for an implant and it really is unrelated to any extraction procedure. Sometimes during an extraction if the bone is thin there can be a communication from the mouth to the sinus but that is different.

Maybe nothing: As the other panel members advised, it just may be a low sinus floor. At times, the buccal bony wall can break away (through no fault of the dentist) and that may also necessitated the need for a graft. But in general, the sinus lift is needed when there is not enough height of bone, and that can come genetically, or pathologically (periodontal bone loss around extracted tooth).