When the gas in the tank sloshes back and forth. The tank in a mustang is nearly the whole width of the car but fairly shallow and sits behind the rear axle. In a slalom, the fuel sloshes back and forth. Now imagine somebody slaping the inside rear corner of your car at just the moment when you're at max cornering capacity on every transition. It's REALLY annoying.

The tank is baffled, but they're not really enough. The baffles are brazed in place and tend to come loose in older cars as well.