The rotten egg smell is sulphur and is caused by the catalytic converter reacting with the sulfur compounds in gasoline. Different driving conditions can cause this.

There are quite a few cars that do this and it is becoming more common as the manufacturers install more efficient catalytic converters.

To reduce the problem they are starting to roll out reduced sulphur content gasoline but it is not available throughout the US yet.

It can smell pretty bad. :rolleyes: