While launching rockets, mine will sometimes randomly explode. This has happened at three unique points for me. At around 50 km up, 100 km up, and in deep space. I have heat damage on, and this may be the cause for the 50 km one, but at 110 km my craft, without taking any heat damage, had one engine blow up when I hit m to go in the map. A similar thing happened earlier, where a large rocket in space exploded without any input from me. It was moving along fine, thousands of km away from any planetary body, and when I hit m, it exploded. Possibly caused by the heat damage option, because it only seems to happen when I have that toggled on.
I have tried several times but I cannot reproduce this issue. I suspect it is related to part damage as well. You can try clicking on parts to monitor their temperatures. Once they start taking heat damage, the Damage property will show up in the inspector panel and once it hits 100 that's when a part explodes.
Also, that's a cool rocket. I had a lot of fun launching it and going through the stages.