A ‘Turtle Back’ can happen for a number of reasons.
Poor overall body awareness, anterior pelvis tilt, leading the lift through the back and upper body, inadequate bracing, disengaged lats and core and one of the biggest issues is a lack of focus of force through the legs on the first half of the lift.
Deadlifts are a multifaceted skill.
Deadlifts are a push and pull skill.
Breathe, brace, engage upper body, lats, pelvic floor and core, push through the floor, maintain position, and then finish it off with a pull to a standing position.
If in doubt, drop the weights and do high volume practice focusing on technique and body control.
Master the basics.
Alternatively you can also explore using hex bars or sumo deadlifts.
Some people die to their levers and current strength levels are probably better dispositioned for these alternatives.
Do what is best for you. Sebastian