I believe a new function called approximate_square_root was used from a library commonly used by steem (the library is called "fc"). It was assumed that this function can only return positive values (like a true square root function). But apparently under some input conditions, it could return a negative number. This negative number in turn managed to trigger a protective check in the code that prevents bad math from happening by stopping block processing.

At first, you might wonder why a "protective check" stops blockchain processing, but it's a good idea to have these checks. These checks prevent generating bad state data or other unintended consequences that could result in money transactions being processed incorrectly.