Yep, no surprise. What you're doing is causing the inductors in the switching power supplies to become saturated and hence the PSU can no longer meet the current demands placed on it = brownout.

This will happen with anything that doesn't have magnetic shielding; EMI shields tend to be stainless steel which doesn't work for magnetic fields... careful though, it could be possible to cause damage depending on how the device you're provoking is designed. Those magnets are freakishly strong!

Hugo