Maybe that wasn't a great course layout.
Although they tend not to be as fast as races on permanent tracks, street course races are certainly exciting, thanks to the sight of race cars on actual city streets. But the reason these races aren’t as fast is because city streets aren’t really designed with racing in mind at all, and it’s at times like the one you’ll see in the following video that this becomes really obvious. The crash actually completely stopped the race, albeit only temporarily.
It all seems to have started as a result of the first car misjudging just how tight a turn it really was, and setting off the kind of chain reaction that you’ll sometimes see when there isn’t enough room.