All of these are international packages going through Jamaica Bay in New York which is notorious for delays. A significant percentage of packages are selected for extra screening here, but no where near 100%. Those go into a set of queues that always mean randomized and often lengthy delays. Most of the packages chosen for the screening are picked by humans that I would almost guarantee are more likely to pick the packages that catch their eye for any reason.

Also, if you send through a package that weighs just north of a kilogram it is almost guaranteed to be subjected to extra screening. This is by no means a secret among international shippers, and would make it pretty easy to stack the deck if you wanted to. I bet a lot of shoes weigh around that much, no?