Thats honestly really weird. The Chukker is basically a beefed up Deep V, which is itself a pretty durable rim... I'd imagine a 40h chukker is plenty strong to handle touring loads! All 3 that failed were rear rims, right? That seems to indicate that its not a wear issue (as the front generally wears faster than the rear). How worn is the rim?

What does Velocity have to say about it? Honestly, my first response is that you got really unlucky with some bum rims. That seems like a very unusual problem.