What are possible reasons these 4 UPS got killed?
Frequency problems, overload, underload or a cobination of those.
The physics is actually much easier than it seems at first glance: Power generators are engines just like the everyday ones we see all around in our cars, lawnmowers, snowblowers, etc.
Except for new power sources like some wind and solar systems with electronic inverters, the vast majority of power is supplied by large rotating AC generators turning in synch with the frequency of the grid. The frequency of all these generators will be identical and is tied directly to the RPM of the generators themselves, generally 3600 RPM for gas turbines and 1800 RPM for nuclear plants. If there is sufficient power in the generators then the frequency can be maintained at the desired rate (i.e. 50Hz or 60Hz depending on the locale).
The power from the individual generators will lead the grid in phase slightly by an amount roughly corresponding to the power they deliver to the grid.
An increase (=defect) in the power load will cause this rotating frequency to drop.
So a battery systems like this is designed to keep short-term fluctuations in power requirements from dropping the frequency because of lags in the governors and generators which require a finite time to adjust to the new power requirements. These "frequency regulator" power stations can supply very high power for short bursts to keep the power requirements even so that the other generators don't see too much load faster than they can respond due to mechanical limitations.
If this does not happen somehow, your UPS will be killed.