Excessive Grease in Above Stove Microwave

This weekend the wife and I replaced an above the stove Microwave that was installed 10 years previous when the house was constructed. It was supposed to vent to the outside.

Over the years I was not confident of the installation as grease would appear in small amounts on the vents of the Microwave. When I took the old one down, I did not note the interface with the outside vent so I cannot comment on that. However, there was a lot of grease on the inside, outside, and against the wall that was hidden by the unit. I was a bit surprised that there was not some sort of fire.

As some of you well know, when installing such a unit, one may have to rotate the exhaust fans on a unit so the air blows out in the correct direction. The old unit was in the correct direction.

The new unit included a damper, that we installed. The old unit did not have one.

So my question is: Is such build up normal over 10 years? Should I conclude that the unit was not installed well and that lead to the build up?

As I said we installed the damper. We also removed all of the old tape from the vent duct to where it would sit on the microwave. My wife (smaller hands then mine) then added new layers of what I can only describe as aluminum foil tape to the interface. She tested, by turning on the fan, that there was no leakage.

Should this prevent most of the build up problems?

For what it is worth, we do eat pretty healthy. However, this house had 6 teenagers in it at one time and often saw dinners and breakfasts with over 20 people. So even having bacon occasionally let to a lot of bacon cooked!