Here's the mechanism: Your spine is meant to hold one continuous, roughly straight line while you sleep on your side — from your skull, through your neck, down through your lower back. When your pillow is too high (which most are), it tilts your head upward and knocks your cervical spine off that line. Because your spine is one connected chain, that tilt doesn't stay in your neck — it transfers straight down, forcing your lower back into a slight compensating curve just to keep you balanced.
The muscles that do that compensating? The same lumbar stabilizers you just loaded under a bar. Instead of shutting off and recovering, they spend the night quietly holding a low-grade contraction. Not resting. Working.
Do that for one night, you might not notice. Do it for a full week after a heavy training block, and "still tight three days later" stops being a mystery.