The tripping of the main on restoration of utility power, can be due to inrush currents of motors starting, as others suggest.
Another reason can be a lot of thermostaticly controled water heaters, hot beverage machines and so on.
Under normal conditions the average load of such appliances can be suprisingly small, since they cycle on/off and only use power briefly once warmed up.
After a shutdown though, all such appliances will draw full load current until the water/space/coffee is heated.
The load can easily exceed the service.
This occured recently at a building I maintain.
Service is 200 amp 3 phase 4 wire 230/400 volts.
Normal load is about 120/150 amps per phase.
Following a utility failure the 200 amp fuses opened on 2 phases, no fault was ever located.
I suspect that the problem was the 28 yes 28! electric water heaters each 12 amp at 230 volts.
With 14 water heaters on each of two phases, the load whilst re-heating from cold would be about 170 amps, that would not leave much for the rest of the building.
