There are some trainers out there who give good advice!
Yes, you need to back off on the weight and perform the exercise correctly. You probably don't need to drop back to 100 lbs though. On the other hand, maybe you will.
In any event you can do bent rows, deadlifts, squats and all kinds of other exercises with way more weight than 160 without rounding your back. It may just be a form issue with you, but you should also make sure you're doing enough work for your lower back.
Everyone has a weak link someplace, but if it's your lower back you'll have problems sooner rather than later