What would cause spasms in upper back and shoulder blade?

Most common cause -- Is a sprain of ligaments in the back, strain of the shoulder blade muscles, pinched nerve from the spine, etc. See your doc..
Multiple reasons. . It can be pinched nerve, radiating pain from gerd, myofascial pain and multiple other reason. Further investigation is warranted.
Several options. Multiple conditions can cause spasms in the upper back and shoulder blade, including direct trauma, sprain/strain, underlying chest pathology, thoracic spine pathology, etc. See a physician if symptoms are severe and/or persistent.