reverse power trip of diesel generator

There are several possibilities. If the voltage regulation settings are not matched, or a regulator malfunctions, reverse power will occur to the lower-regulated genset. A fuel delivery problem to one genset, such as clogged fuel filter, will cause it to run slower, and it will then receive reverse power. Similarly, a governor malfunction or incorrect setting could also cause one genset to run slower.

Some preventive measures include careful matching of all settings, consistent upkeep of fuel filters, and prevention of air leaks into the fuel system.

Well

My first thought is the gen that tripped had a different voltage/impedance than the others which in turn the other gen's start supplying power to the lower voltage one.

if the avr and governor are working ok and fuel is intact reverse powers is very very rare