I have this same issue.
I have this unit in my Jeep. When I have the top off, I always detach the faceplate when I get out. Sometimes when I drive to work in the mornings with the top off, it is cold, so I blast the heat. The vents are right below the unit.
Every time I blast the heat and then try to remove the faceplate, I have this same issue. When I try to remove it after NOT blasting the heat, it works fine. So I am assuming the motor gets too hot or there is some type of temperature protection.