I've not seen the movie so can't say why.
Maybe the T-rex is Chomper as an adult. Maybe while he patrols outside the great valley that is one way he keeps himself entertained.
Just wish I were smart enough to understand all the lyrics instead of the odd word here and there. Nice music though, and same for the beat of the song.
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I removed the offensive link above since I heard from someone on another forum that it contains some non g rated and nasty lyrics kids should not hear. I could not tell since I can only hear the occasional word and I think a part of a sentence something like, "go to town.."