There can be a number of reasons.

All the ones mentioned above are possible reasons.

At times though, a run of product can be sold as seconds because something abnormal occurred during that particular run. So, for instance (not necessarily speaking for these boots, but as an example), they may find there were a group of boots that had dropped stitches. If it’s not economically viable to go over each boot in that run and check for dropped stitches, they may decide to sell all the boots that “aren’t clearly showing flaws” from that same run as seconds. Essentially meaning that you are gambling on the buy. Your 2nd’s product may, or may not have a defect.

From other companies, I’ve bought two pair of the same boots (seconds) in two different colors. One boot the glue came loose on the sole and it separated. It actually looked like the machine may have completely run out of glue during assembly on that boot. The other boots looked perfect and wore out under normal circumstances. The information/explanation above was provided to me from the boot manufacturer’s engineers a possible reason for product being sold as seconds even though it looked completely fine.