Not sure if this is a new bug, but never happened to me before. I presume it has to do with me standing on the edge of the wall, but still. The only options to leave the arrow cart are waypoints, logging out or maybe being knocked/pulled.
Ah ha, same thing happened to me with a catapult. I thought I was lagging or something. I had to logout to the character select screen to get out of of WvW and come back.