It's me, hi, I'm the problem, it's me (sorry, I think I'm the one who sorta derailed the other thread with overhead bin comments. It was slightly related but also slightly not).
Anyway, my backpack always goes under the seat in front of me, regardless of whether I put a small roller bag in the overhead bin or I have a checked bag. It's primarily because all my 'in-flight' stuff is there (tablet, snacks, water, etc.) so I want easy access. And partially because it fits, so that leaves more room in the overhead bins.
I fly SW all the time, and they are constantly repeating during boarding to put smaller items under the seat in front of you and leave the overhead bins for the larger items/roller bags). So that's probably been drilled into my head too. lol.