The circuit takes your mains AC and put that through a diode bridge that rectifies the AC into DC. Due to some AC ripple still exist a capacitor bank is inserted to remove that ripple. Then the DC is converted into high frequency AC for the step-down conversion.
So, either the frequency becomes so low that the capacitor bank discharges too much and thereby creating a low-voltage condition or the generator protective circuits are kicking in creating a brown-out in mains power... But any 'international' power supply should at least cope with frequencies down to 45Hz.
A cool thing is that most switching power supplies can run on DC mains input, even if it isn't highly recommended