I got two points on this...

1) Anyone ever notice that if you're in the cockpit that initiates the wave and the other cockpit just stares or doesn't even see, the series is followed up by "A-hole"

2) I've only really noticed this at ORD, but I think it has something to do with the inbound and outbound ground freqs. Anytime there is another airplane converging with yours on the ground, opposite directions, or converging at intersecting taxiways or just taxing somewhere in general, the captain always sees them and says almost verbatim, "Aw, whats this a-hole doin'?"