Conventiinal Deadlift, losing tightness in my back?!

Hi,

I think I have a problem with my back-tightness on my conventional deadlifts.
Every time I pull over about 180Kg I lose tension.

Do I start with my hips to low?

https://m.youtube.com/watch?v=v3I-5_HLudA

The weight on the bar in the video is 190kg.

Thanks