convection microwave that also broils
Generally speaking the same element is used for both broil (grill here in the UK) and convection.  In the latter mode a fan distirbutes the heat evenly around the oven, but for tall items in particulart the top heat from the element can be a little fierce.  I've got 2 Panasonics that work this way, and when investigating those all major rivals seemed to be the same.