That sounds like spark plug 'blow out'. Usually means the plug was incorrectly torqued. If it had of been ejected it probably would have stripped all the threads, so luckily that didn't happen.

There's plenty reading on the web about it and the causes for it to happen. There is a mention of ignition timing also, so maybe get that checked out too.
One such article: https://www.carsdirect.com/car-repai...roblems-solved