People use things called botnets (basically compromised computers, IoT devices, etc, which can number in the thousands or millions) to send potentially billions of requests a second to a website. The site can't handle the load, and basically tells the owner it's going on strike. The site goes down.
As for why it's happening here, who the hell knows. No idea why someone would risk jail (or worse depending on the country), to inconvenience a few hundred baseball fans in Canada.