Basically, a recording of his came out that had the lyrics to "Baby Got Back", but against music that was entirely fresh and original. And then later, Glee had a scene with a (really weird) a cappella group that did a cover of Coulton's recording - his music, but with the Baby Got Back lyrics.
The show gave Coulson zero credit, didn't ask his permission, didn't give him any compensation, and I believe didn't give him any notice ahead of time. (I'm typing this all from memory, so I'm sure I might have some details wrong, but that's the thrust.)
So the question is, should they have? It turns out it depends entirely on how the creation process was described.
There's an old songwriting exercise where you take a song, and then write entirely different music for the lyrics, and then write new lyrics for the song. Presto, it's a whole new song, and there's nothing wrong with that songwriting process. You can do the reverse where you write new lyrics for the old music, and then new music for the new lyrics, too.
Coulson could have easily written that new music for "Baby Got Back", and then written new lyrics for it, and copyrighted the song. And then, I'm not sure, but he possibly could have then released the version with the "Baby Got Back" lyrics as a joke/parody of his song, and it still would have been his song. But since he released his version as a cover of "Baby Got Back", all Fox had to do was "steal" his version, pay the royalties to Sir Mix-A-Lot, and cut out Coulton entirely.