When I backed the game, there were only two backers.
I expected to be the third, then it crashed.
When I received the notification 5 minutes later, I was around the 229th backer.
So I guess those 227 backers at once + everyone else browsing at that time made the system crash.
I saw the same thing and was impressed I was going to be so low in the backer count. Then I got my confirmation email and was backer 1153!