you guys have already mentioned it and are all over the cause of it. its not your steppers. it all this boils down to resonance from sudden direction changes. almost every machine does it to some degree. some are better than other but this is what core xy movement gets rid of or atleast limits. unfortunately printers with core xy are extremely limited but you will be seeing it more and more on newer printers being released. this can also come right down to the material your printer is made of, how rigid the construction of the frame is and if the bed is cantilevered like on a mb/ff and tons of others and also the weight of the moving print head. you can lower the acceleration which can help a ton but this can also just be inherent in your printer's design and you will never get rid of it 100%.