If she is doing this only at shows, then it is probably a nervous reaction; she knows the trailer is a 'safe place' associated with home, so that is why she wants to return there.
Perhaps take her to a couple of shows and just walk her around on lead, perhaps pony her around, if you can. That way she can get used to being in a different setting like this without being too stressed out. Then the next time, you go, just ride her around, but not show. Maybe trailer to show grounds when there are no shows...I know for our horses this has helped greatly; you can't always expect every horse to take to showing really well, without desensitization.
I also agree with the putting her to work when she gets antzy...trying to keep her still will only frustrate her, and can actually cause the rear to happen; if you can keep her feet moving, then ask her to stop and relax after a bit of work, she will eventually learn that standing still is okay.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."