Looks like the driver became distracted, then lost control. Sad story. When I see these I like to remind myself of how quickly it can happen. No mechanical failures from what I read.
https://www.nbcnews.com/news/us-news/new-york-tour-bus-crash-victims-identified-rcna226776 (https://www.nbcnews.com/news/us-news/new-york-tour-bus-crash-victims-identified-rcna226776)
Kind Regards, Phil