Can be for any reason, whether it's music, animation, content, etc.
It's certainly not the worst ever, but I would say the second season of Shokugeki no Souma has a pretty shit opening. Half of it is just still frames and title cards, while the other half is... people riding on knives, I guess? And a little bit of out of place fanservice?
the baccano ending theme is incredibly lame both animation and song-wise, especially considering the show's fantastic opening.
https://www.youtube.com/watch?v=9X1_umbDuvI