It was a glitch that got turned into a gag.

The aggressiveness of nations was on a scale of 1 to 10. Lower is peaceful, higher is aggressive. Peaceful nations try to get democracy, which decreases the aggressiveness even further. By 2, to be exact. Due to the code language, 1-2=255. This results in Gandhi being a bloodthirsty maniac with a nuke fetish.

"We're not trying to keep people absolutely accurate. They're still going to do random things. We're still going to make them mad, and they're going to do things uncharacteristically, because that's part of what makes Civ fun. If you're playing against Gandhi and he doesn't declare war randomly on you at some point, then we haven't done our job." -dennis shirk.