Physio pressed on thoracic vertebral fracture causing immediate wheezing. why would this cause this? (fracture not dx until later)

Hard to say: Wheezing is usually in some way related to reactive airway disease of one form or another. Pressure placed on the posterior aspect of the thorax could cause air to be expelled from the pulmonary cavity and could, in theory, make the wheezing more apparent. It is possible, especially if you have osteoporosis, that the fracture occurred at that moment as well.