Why steroid cause cataract?!

Natural lens is crystal clear due to many intracellular mechanisms and enzymes that inhibit oxidation. Steroid will act against these enzymes and inhibit them which will keep the lens vulnerable to oxidation process that will lead to cataract formation.

http://www.online-eye-info.com/what-causes-cataracts.html

Because corticosteroids can increase intraocular pressure, leading to the premature formation of cataract. The risk is increased with long term, high dose CS.

you have cause and effect the wrong way around.

It's actually cataracts that cause steroid