A. The drum symbols appear when the RHYTHM button has been selected. If the rhythm button is held down for a longer period of time, this will switch on something called 'One Touch Setting', which is where the piano automatically chooses a tone to fit the selected rhythm. As the choice of tone is automatic, this may be the reason why the piano changed to different sounds other than those in the registration memory.
The one-Touch mode should automatically switch off when you turn the piano off.