>>What caused the second explosion then?<<
Good question. Nobody knows. Ammunition doesn't seem to hold water, and coal dust doesn't hold up well either. I suspect a boiler may have been ruptured, but nobody really knows that as an irrefutable fact as either. The problem with the boiler theory as understood is that nobody can seem to find any examples of a Scotch Marine boiler exploding in such circumstances. I suspect if one blew, it was as a consequence of torpedo damage.
>>BTW, a coal dust explosion could also happen on the Carpathia...<<
Only it didn't.
>>so was the Lusitania plain UNLUCKY or was it something else?<<
More like something else. See what follows.
>>It also seems stupid for the crew not to order portholes shut in the war zone when they have the sense to put out lights and swing the lifeboats out...<<
Or have watertight doors set properly perhaps? This might seem elementary and in some respects it is. If you go cruising through a war zone, it makes sense to have things buttoned up as far as possible. The catch is that while this makes perfect sense to a Navy guy like myself, the message doesn't always sink home with a merchent crew. They don't have the day to day experience in setting watertight protection conditions that the crew of a warship does and wouldn't always think much of opening a door, and then forgetting to close it thereafter.
A door open here, a door open there...for want of the nail, the horse, message, and war were lost and all that. Those little things which ones doesn't think of on a daily basis can come back and bite. The ship wasn't killed by the mysterious or any grand conspiracies.
She was killed in the end by mundane carelessness, and it happens a lot more often then a lot of people know.
Nothing inexplicable about that.
(And yes, I could be wrong.)