I'm throwing some stuff out here because I have this problem, too. The fact that I spend the majority of my time in a chair hunched over in front of a computer not withstanding, I think it is due to a 'load order' error when I squat.
There is a mobility wod video discussing it, but my working theory right now is I use my hip flexors to over-arch and keep a too vertical back angle. I liked it initially when I started, because it kept me tighter out of the bottom, but eventually it led to a lower back tweak due to the upper attachment of the illipsoas or psoas or whatever it is.
I figured it out setting up to DL, when I started getting set up and feeling a pull/tension in the tweak spot in my lower back. I was able to make the adjustment and load my glutes and hamstrings first, before my hip flexors and continue onwards.
Now, I'm working on getting a little more forward lean in my squat, while still staying tight and I don't feel any agitation after squatting outside of the usual soreness.
Just a theory as of now.