Microwave micro switches
Typical microwave ovens will have multiple micro switches that are designed to provide safe use of the unit. Safe use translates to making sure the door is closed before the unit is able to produce the microwave energy. 

The safety feature is normally designed so that the multiple switches have to sequence on/off properly when the when the microwave door is opened and closed. If the switch sequencing fails (broken mechanism, misaligned switches or user attempted defeat of safety feature) the wiring is designed to place a short across the AC power line in the microwave downwind from the fuse. The result is that the fuse blows and prevents operation of the oven in the unsafe mode.

In your case the problem is likely to be one of the other switches has failed or the interlock mechanism that triggers these switches is broken or misaligned.

You may want to reference this older question and additional links therein:

Microwave door interlock switch keeps failing

Pay particular attention to comments regarding OEM replacement switch costs versus buying switches from an electronics supplier.