What would cause bleeding after a transabdominal ultrasound for previa?

See doctor. Did the ultrasound confirmed the diagnosis of placenta previa? If so, the bleeding is most likely due to the previa itself since the ultrasound was a transabdominal one and not transvaginal. If you have placenta previa and are having vaginal bleeding, please see or call your doctor immediately, as it may progress to preterm labor, and you will need a c-section and other emergent medical care.
Not the ultrasound. Neither transabdominal nor transvaginal ultrasound increase your risk for bleeding. Contractions and/or cervical change can result in bleeding with a previa, but most of the time, there is no apparent reason. The placenta has a huge amount of blood flow through it and with it positioned over the cervix, bleeding frequently occurs.