The AC startup draw is more than the generator can handle with the other loads that are already on. Startup amperage/wattage is usually double what the 'running' amount is. For example, if you have a 3 KW generator, and the A/C draws 10 amps running, but 20 amps to start, and you are already using 15 amps for other things - lights, computer, TV, stereo, fridge, chargers, etc. The total amps you need to start the A/C with the others going is 35 amps, or about 4KW, since the generator cannot supply enough power, it becomes overloaded, then shuts off to save itself from burning up.