3 Months ago in BWI we were told by ground control that our sequence was the A320 (I think a Cactus, but I can't remember) and to follow them to 15R. As we approached a merge in two taxiways we saw two SW jets waiting for the A320 and us to go by. As the 320 passed in front of them the first 737 "gunned" the power and cut us off. We slammed the breaks on, throwing our FA's against some seats as they did their demos. As we started to taxi again, the second SW jet did the same thing. We questioned ground (who had given everyone their sequence) and after a LONG pause, the controller came back and said "Oh, I guess they changed the sequence". "They" must have been that other controlling agency who gives contradictary clearances from the actual ATC controllers. This was a dangerous maneuver which could have resulted in injuries to our flight attendants and/or a collision. Yes, this kind of thing does happen.
Now, I don't think this is something SW tells their pilots to do to gain a couple minutes in the departure line. Rather, I think a couple of "cowboy" captains decided on their own that they were tired of waiting for some sequence they didn't understand. (I wish I knew why ATC does everything they do). Every airline has its "cowboy" captains. So to say this doesn't happen is ignorant. I just don't think it is a conspiracy by an airline. 99% of the pilots flying are phenomenal. Unfortunately, the 1% sometimes ruin it for the rest.
Cheers.