User blog comment:IamJakuhoRaikoben/Chapter 455: Magnolia's Defensive Battle, Review./@comment-24546844-20151005142615/@comment-7418318-20151005144706

Interesting theory about Brandish from which follows: WILD SPECLULATION TIME :D

Your comment, together with something that Dan Straight did to Angel made me think this scenario up:

Layla was weakening, because somehow magic was harming her body to the point where she couldn't be a mage anymore. She kept wasting away until she eventually died.

Could it be that Layla was having too much magic on her body, to the point that her body couldn't handle it anymore and was damaged by it? We've seen a similar case with Ultear, so it is definitely a possibility.

Now Lucy might have inherited this too, but with all his money Jude found a way to kinda "shrink" Lucy's magic to a no longer dangerous level. Since her magic might work in a similar way, maybe Brandish noticed exactly this shrinking.