I personally like the theory earth gravity changed due to shift in our planet’s orbit
Gravity shift is a possible cause and has some evidence to support it - right now gravity on Earth is lessening in various places - no one knows why.
No land animal larger than an elephant can exist today, else it would be crushed by its own weight and bones of calcium would be too weak. Yet there was a class of animal which, in many cases, was many times larger than animal today and could not exist under Earth’s current gravity regime.
Another unanswered question is what happened to all the Oxygen? During the rein of the dinos Earth’s atmosphere had a much higher oxygen content - where did it go?
The Paradox of Large Dinosaurs
Applying Science to Understanding Large Dinosaurs:
http://dinosaurtheory.com/big_dinosaur.html