Could be anything. There are lots of diff reasons. For example once he got out he could have been mad and torching everything in site or he couldn't control he fire powers in an enviroment( the Dragon King nesting place was heated so he was sort of at home with all that heat and lava), or he could have done it himself, he could be evil and just testing he powers for the 1st time in a outside envirment just to see how powerful he can be on real life objects. The stuff above are only my opinions.

i think blaze is evil..cuz if u listen to his ending.it says "He then resumed the quest he had undertaken before being subdued by his captors."..which means he was all ready torching all that ish to begin with.so i dunno whut the deal is with him..maybe he has an anger management problem

Hmm, interesting i've never seen his ending.