It sounds like your dog has become "ring-wise" and developed a habit based on something that happens only while at a trial. The most common form of this is poor 2-on-2-off performance because in a trial handlers are so focused on getting that Q that they let their criteria slide which leads to broken stays, but only in a trial. Dogs are very smart and can figure this stuff out.
When this happens, you probably want to take a step back from trialing until you can stop this from happening. There's no point in spending the money when you know your dog is going to rush to the leash. Here's some things that I would work on:
During practice, try to emulate the trial atmosphere as much as possible. If you can, try going to some fun runs. They won't count for anything but will be cheaper and more like a real trial. If you can get him to mess up and go to the leash not in a trial, think about what was different about the environment so you can work on it.
Work specifically with the leash as a distraction. There's probably a ton of value in it since it's the last place your dog goes in a trial and he probably gets a huge reward, not to mention the leash means walks, agility, and all kinds of fun stuff. Make sure he knows he's only to go to the leash when told. Work with the leash on a leash stand like the ones at trial too.
It sounds like for your dog the key is going to be emulating the trial environment in a practice setting as much as possible so your dog thinks he's at a trial.
Alternatively, you can ask the leash attendee to hide the leash somewhere else at trials so it's not a distraction.