I think it comes down to how immediately recognizable some aspect of a piece of music is.

If someone hears a song and they say..."that reminds me of...." ....Probably not a big deal.
if someone hears a song and thinks that it is another song and then finds out that it is not, I think that draws a clearer line on theft.

And then other things come into play...the Marvin Gaye copyright holders may have never had a case if Robin Thicke had not remarked about the lift.

I personally think that part of it is that the theft has been so blatant and "in your face" by the big dawgs in the music industry fr so long, that the worm finally had enough and turned.

Disclaimer...all of the above is the opinion of a hobbyist who knows nothing about copyright law.

https://soundcloud.com/sunset-poet-of-texas