ubercart - many "in checkout" orders
In checkout is a stage when someone adds something to the cart but does not go through the checkout process. It has nothing to do with the way your site functions. Just the user didn't want to checkout.

This problem (most likely) is Shopping Cart Abandonment (Google it).

UC Followup is a module that helps users push through "Adondoned Carts".
http://www.drupalmodules.com/module/ubercart-followup