What could have caused these step formation cracks in a plaster wall?
It could be settling. I see from the window sill it is on an outside wall. You did not mention how long the cracks have been there, but I would go outside and check if there is any water damage happening at the foundation level, missing siding, etc., to rule out settling from decay. Otherwise those cracks, I would say are normal in many cases for a wood lath and plaster home, which should be the case for your 1929 home. The cracks follow the typical way the wood lath is installed.