What would cause spasms in upper back and shoulder blade?

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.

Multiple reasons. : It can be pinched nerve, radiating pain from gerd, myofascial pain and multiple other reason. Further investigation is warranted.

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..