not sure why. guessing it's wireless interference of some kind.
Kitchens are full of devices that are hard on WiFi.
Microwaves are by far the worst offenders as you've found as they emit a lot of 2.4GHz radiation. Spending a little extra on a microwave can help 'cos mostly the issue is poor shielding and, on cheaper models, an especially noisy 2.4GHz waveform.
Cordless phones commonly use the 2.4GHz band as well, and this can cause issues.
Sometimes just flipping the channel that your WiFi network uses will clear problems.
OT: Personally, I went in completely the opposite direction and set up a cluster (four at present) of AEs in my basement, and wired them direct to the router. Four auto-sensing amps are connected to the AEs, and speaker cabling is run to in-ceiling speakers throughout the house and pool area. I also have an aTV in the living room, so I can use my phone to play music to pretty much anywhere in the house, without needing to turn amps on or off.
It's a shame most people have no idea how useful AEs are. As a networking device, they are fantastic little swiss-army knives that can be used to easily solve a whole slew of common problems.
As a home audio device, they rock. If I'd have used off the shelf whole house audio components from Bose, or any of the standard home automation companies such as Control4, I'd have spent easily three to four times as much as I have, for less utility, flexibility, and far more lock in than even Apple manage to exert.