I am not here to speculate either way whether this situation is based on a common cause. I am thinking about this from the other side. What would you expect to see happening by chance? Random systems do throw up clusters. You can, if you have sufficient data, look at the apparent clusters and calculate whether you are seeing more clusters than chance. One cluster on its own tells you nothing. The fact that kids in the street contract the same illness may be due to common cause or it may be simple random bad luck. If you are there on the ground and you understand what might cause such illnes, then you can look whether there is a causative factor.
We often misunderstand chance. "Number 27 came up in the last three draws of the lottery." People will see this and decide to avoid the number (that is if they throw their money at such things. In a typical lottery you might expect to get back half the money you stake. I don't "play" the national lottery). Chance says that if you toss a coin and you get tails, on the next throw you will have a 50-50 chance of heads/tails on the next toss. If you have tossed tails 3 times in a row, the chance on the next toss is still 50-50. If you have tossed tails 10 times in a row, the chance on the next toss is still 50-50.
If you keep getting tails, then you might get suspicious that the coin is biased, but 10 in a row is not in itself non-random. Neither is 20 or 30 or 40. But if someone bets you in advance that they will toss 10 heads in a row, I would suspect they had a biased coin. But if you keep tossing a coin you will get all sorts of interesting patterns.
In a big place like Canada, you can expect patterns popping up. Nobody writes to the paper saying "We haven't had a cluster here". Even if they did, would it get published? I don't know enough about MS to know if there is a infectious or environmental factor that might cause clusters. But it can be simple bad luck too.