I blame dinosaur farts
More that you know.
It has been speculated that one of the reasons that the big herbivores got so big was that given the apparently poor quality of their food, that they didn’t chew in many cases, they used fermentation based digestion.
As with all fermentation processes the bigger the still the more efficient it is, so in this case it was a big efficient stomach ... which in turn means a big animal to house it.
Now, cows (who chew their food) use fermentation digestion and there are (relatively small compared to dinosaurs) cows that can sustain a flame ... just imagine what a big dinosaur could do?