Hi Terry,
It's likely that the motor is over worked due to the added torque it has to overcome when pressed down, that could result in it cutting off power to protect the motor.
It's warned in the [link|https://cdn.makitatools.com/apps/cms/doc/prod/BO5/4d49e26d-a944-4267-82f6-64cafff90b25_BO5031K_IM.pdf|manual|new_window=true] to not put more than light pressure on the sander:
[image|3529244]