From people I know in the trucking business, the problem with delivering to Amazon's warehouses is inefficiencies in getting the trucks into and out of the yard. They're just not being run well. Stories of 50-truck-long queues being run off by the local police for blocking entrances to other nearby businesses..

Hopefully this will help, as the trucks can deadhead[0] to their warehouse and do a drop/hook[1], with the trailers being positioned to the warehouse doors by Amazon's own employees acting as yard dogs[2] for loading/unloading the contents. This lets Amazon manage the activity in their yard themselves and hopefully be more organized.

[0] Drive just the truck portion, without a trailer.

[1] The cab (only) portion of the truck drives to where the trailer is, hooks up and drives away with it. Upon delivery, they park the trailer in the yard, lower the landing legs and unhook.

[2] A yard dog is a specialized truck that is shorter and more maneuverable, able to make tighter turns in the confined spaces of a yard.