Dc power supply outputting sine wave at both terminals
The output is isolated and floating,  so it has very high impedance to earth/ground.
The oscilloscope inputs also have very high impedance to earth/ground, and the setup is a differential measurement where scope earth/ground is not connected to the circuit, so the strongest impedances that connect the supply outputs to earth/ground are the scope probes.
Any external disturbances, even small ones, are able to move the isolated output voltage with reference to earth/ground.
As the power supply has most likely a standard mains transformer inside it, even a weak capacitive coupling like for example the few picofarads of stray capacitance between transformer primary mains side and isolated secondary low voltage side can make the isolated output to have common mode voltage to be few volts of mains frequency sine wave in respect to scope earth/ground like what you are seeing here.