I've had that problem with an older car and that was because the air filter needed to be changed out. With new cars, I assume it's because the deposits of sulphur and hydrocarbons that builds up in the catalytic converter are being burned up. Heard this happens when you usually drive your car slowly and the odd times when you put the pedal to the metal, it burns up the deposits built up.
This is just my guess.
This is just my guess.