Question: It's a common practice to highlight an element on hover state. 
By highlighting it, I obviously mean making it more visible than other elements.

But lately, I've witnessed the opposite trend on a couple of websites : 
Opacity is set to 0.5 or less on hover, which makes the hovered element less visible instead.

To me, this is nothing but a bad practice.

Or is it a new trend backed by some well known data that I'm unaware of?

Answer: 
The purpose of the hover state is to give visual feedback to the user highlighting an interaction opportunity, which does not necessarily require a visual highlighting.

While it has commonly been implemented in a way where a solid and prominent color was lightened on hover, this is not the only valid use. The appropriate use will depend on the context and other interaction elements in view and their relationship.
I agree that it is **currently** a bad practice, primarily because it is returning to the days of metaphoric design. The designer is trying to inspire the appearance of (and a realworld reference to) a frosted sheet of glass floating above the surface of the rest of the interface.

In this age of Modern-UI, it is common practice to avoid metaphoric/skeuomorphic elements in our designs.

From a purely UI standpoint, I don't think there is anything wrong with floating sheets of frosted glass. They are pretty in a nostogic kind of way... and they do focus the user's attention on a subset of input options which is what they are supposed to do. 

They just are currently not popular; non-standard enough to be considered mainstream yet still too familiar to called revolutionary. Just a little too... dated.