Check the order status right before this unexpected redirect happens. Is it "pending" or "in_checkout"? If it's "in_checkout" you'll be fine. If it's "pending" then you might get redirected back to the cart. What status is your module leaving the order at before the handoff back to ubercart?

If you have the workflow action enabled to change your status to "payment_received" on a zero balance, make sure that the in uc_cart_complete_sale you are getting the true status of the order in the database before the switch to "pending", not the one from the $order object which can be out of date depending on the order you call the payment workflow and complete sale. If the payment workflow is called before complete sale, the status setting can get clobbered. Lastly, look at the order log -- see anything funky related to statuses getting overwritten. I don't know about your module, but these are things I've noticed with another payment module.