Our top video has 440,000 views ( 4 year old builds first home Little Tikes Playhouse )... It took ages to get to 100,000, but when it did it flew up to 400,000. It's kind of slowed down now, I guess bigger and more popular channels have done the same thing since and their videos come up more in suggestions these days.
Reason for success, the title and idea perhaps. we don't know really. But the most successful videos are usually that popular because of how YouTube promotes it, or ranks it, how much exposure it gets.