I honestly never new this, I feel like a Dumbass. From destiny reddit :
I just discovered this the other day, and I haven't seen anyone talk about it before so I'm going to go out on a limb here and posit that this isn't that widely known. If you crouch behind cover, you can zoom/aim and you'll pop up from behind cover automatically, returning to cover as soon as you let go of zoom. Example here.
The caveat seems to be: you need to be stood immediately behind some cover that you can see (clearly) over when stood up, but which covers your view when crouched. Very useful for certain boss fights (e.g. Valus Ta’aurc).