A safety check may be in order, never a bad thing, but I believe it is not uncommon for quite healthy microwave ovens to cause interference to wireless lans.
A problem is that, grossly simplified, the regulations for 2.4 GHz band essentially require equipment to be able to accept interference, rather than not to cause interference (though there will be other regulations that effectively reduce potential for interference). Such equipment includes microwave ovens, radio controlled toys, smart meters, etc. All can use 2.4 GHz, and so all can cause interference for WiFi, all perfectly legal.