Matatus are supposed to continue further up Tom Mboya Street than where this one has stopped and queue up with the rest of the matatus to wait their turns to be filled in an orderly manner. But desirable matatus stop early, well before the line up. They don't stop for long; you jump on quickly or get left behind, and then try to get past the official queue of matatus awaiting passengers further down the road without getting hassled by the matatu minders. That's what's going on here - "Party" is popular, and can empty and fill long before it hits the queue, making more runs, and therefore more money. So it pays to have those subwoofers.