Having also worked at UPS doing the work hodenkrieg described, I can tell you it isn't "at random" but rather by number, lowest to highest. There's shelves on each side of the back of the truck, and the lowest numbers go towards the driver, highest to the back door. Shelf front right near driver are the 1000-1999, shelf front left are 2000-2999, shelf front right middle are 3000-3999, and so on. 1000 will be right behind the driver at the end of the shelf, 1001 next to it, 1002 next to it, and so on.

But like he said, it's insane the number of packages they stuff in these trucks, especially this time of year. You can very quickly fill up the spot for the 3000 parcels, and still have more come down the belt. It's gotta get on that truck, so you start to use the 1000 space. But it's random when the actual parcels show up, so perhaps at the end of the day you get swamped by a bunch of 1000 parcels, and now you don't have enough room in the correct spot, so you find somewhere kinda near where it should be and stick it there. Most of the time it's still close enough to where it should be that the driver can find it and not have a problem. Sometimes it isn't.

See, it isn't just the drivers who are timed to the second. The guys loading the trucks have 3 seconds to put a parcel in its right location in the right truck, and move on to the next one. They're loading three trucks at the same time, each might have 500-1000 parcels in it by the time shift ends. If you take longer than those 3 seconds, shit piles up on the slide and a super comes over and rides your ass. So when you've run out of space and are having a bad day, you stuff that shit wherever you can find room. The next day when the truck is loaded up again, that forgotten parcel from the day before is going to be one of the first one in, and is going to be in its proper spot.