I have had the same problem with my 2009 bar.
Over about 2 years with the same leash I have riding as per usual and just found the leash trailing behind me on about 5 occasions. I have recently tried using a naish leash, it has a different style clip but it still happened once.
I connect the leash to the ring under the chicken loop and the clip is functioning perfectly.
I think the problem is caused by the leash attachment ring being rigid, if the leash flicks around a certain way it is possible for it to unclip itself. Its possible to demonstrate this by manipulating the clip by hand and it will disconnect without too much effort.
I believe this could be fixed by having a flexible point to attach the leash, like a loop of rope instead of the fixed metal ring.