Actually, velcro is not an ignition source. It cannot cause a spark.
It is however a potential fuel, and aboard the Apollo 1 spacecraft, it, and several other materials became a fuel, once an ignition source was manifest.
The ignition source was electrical arcing, caused by frayed wired most likely inside a frequently accessed panel located in the right side of left hand lower equipment bay. Also present in this area was a leaking glycol coolant line, which seemed to be the initial fire source.
This incident was the fault of everyone involved with that spacecraft, NASA, North American, etc. This has already been clearly established. One doesn't need to believe it. It is a fact.