Level 9 Valued Member
Elite Certified Instructor
Hard to say without seeing.must have done something wrong!
It's so easy to video these days with a phone... I'd recommend you video yourself each session, and even if you don't every upload or keep, you'll have something to look back at if your back feels better or worse later.
I watched the videos above again for any clues, and this is really just a guess... but I notice a bit of tucking the pelvis under at the top of both the swing and the deadlift. I'd practice moving from hinge to plank slowly without doing that. Put your fingers on your hip bone and thumb on your ribcage when standing in a neutral position. Move to hinge, then back to plank, and back and forth with different speeds. The distance between fingers and thumb should not change. If you tuck your pelvis under, it will get shorter as you stand (not what you want, but possibly what you're doing).
In the standing position, you want TIGHT abs and TIGHT glutes, but no pelvic tilt.