Some common reasons are :
1). Load applied to generator exceeds capacity, typically would cause machine to slow down, drop frequency and voltage typically cause exhaust black stacking and depending on the overload and protection settings could cause machine to stall.
2). AVR Voltage gain set too low to respond to load being applied.
3). If AVR feedback is from generator bus VT and voltage dips too far, all excitation could be lost and would typically result in an undervoltage and overcurrent trip.
4). Speed control gain set too low to respond to load being applied.
5). Mechanical issue such as failing turbo unable to provide enough air of high enough pressure while taking on load which will cause machine to slow down and in some cases stall on high load application with sufficient reduction in speed voltage will also lower as well as frequency. In this condition you'd probably also see exhaust black stacking.
6). Mechanical issue such as fuel injection clogged or fuel filters clogged causing the machine to be under fuelled to handle load application and may cause machine to slow down, which in turn would drop Hertz and Volts.
7). There's a whole truck load full of other reasons why this could happen.