Lets be honest you probably dont know what that classical musical reference means (neither do we).
Is there a memo, perhaps, that dictates that the Bachiosaurus gets the big hit of sentimental accompaniment?
Yes, but no its really just a well-utilized realization employed by Giacchino and his predecessors.

Universal Pictures; Inset: Maria Giacchino
Its one of the most nostalgic moments for people who love the franchise, Giacchino notes.
Every time you see a Brachiosaurus, theres that nostalgia button thats pushed.
So, spoiler alert, but the soundtracks fourth track, Nostalgia-saurus, probably isnt about a T. Rex.