The signal appears to be digital so it looks like the noise is within the sampling margin. In other words, suppose at 200mV/division and 8-bit per sample the full scale deflection is +/- 410mV, the quanta would be 410/127 = 3.228mV. If you zoom in 120 times you now have 1.5mV/div so each quanta now takes two squares so 2mV of random noise looks like a squarewave.