It's a problem everywhere...initial build many of the "sparkies" don't label their circuits very well. And then after that, your breaker box becomes a total mess..."I need a circuit here so I'll just steal this one". Hope they "capped" your 240v line in the box rather than "snipping" it...you might want that line someday.
Why energize the panel until the ends are secured? Sounds like a recipe for disaster there but I'm a mere amateur that knows too much about wiring. Guessing when that "Pow" happened the air got nasty and a few people excused themselves to the bathrooms.
I think he hired a pro and already knows what happened. Guessing somebody needed a new circuit and "re-arranged". My elderly MIL bought a house close to us not long ago...not that old but I guess the original owners were cheapskates...since the original build, the house added a "breezeway" and installed central air on that original tiny panel...that box is now a mess. When I built this house I buried a line to the shop on a 100a breaker from the house. No "re-arranging" on the house side there! On the shop side I have a 20 slot panel...will never get filled but I don't have to worry about what I do in the house.
One of my best friends has worked for the power company for 30+ years...at the time included building the "tower" stuff. He has a lot of stories to tell (still don't know squat about residential wiring though...go figure).