Macbook pro 2018 with Mojave overheating when connected to external monitor
The higher power GPU in some MacBooks Pro puts out a lot more wattage than the low power GPU. 

The reason it gets so hot when hooked up to a monitor, even when you’re just running Word or Safari, is because the display outputs are hardwired to the dedicated GPU - if you’re using an external monitor, the dedicated GPU (graphic card) is in use regardless of how demanding the workload is. This puts more stress on the cooling system, resulting in higher-than-average temperatures ;)
Thats the reason. I tried all my 6 Macbook Pro's and all does the same behavior. Switching to the dedicated graphic card and you can't just use the integrated iris.