It also could have happened instantaneously. Performer kicks instrument, knocks it over, instrument breaks, sparks and sets fire to the curtain.
It obviously wouldn't pass NY fire inspection if the curtain went up as quickly as it seems to have. That's my question: Why isn't the road house getting a finger pointed at for the curtain/the circus for having an unsafe, non-fire proofed curtain? You can blame a performer, or lack of an ASM if you want, but there should be redundancies in safety like fire proofed curtains in case a person can't be around that area of the stage 100% of the time.