Not a chance it is coincidence. The only common thread may be usage of some cloud service provider though I don't think there is anything common between them all.

I would be curious if other popular tech/programmer sites are under attack and just thwarting it better?

The common in-between is the users. I hope this isn't the case, but if hacker was able to get access to any MongoHQ db there probably is so much time needed, until you find a db holding full-text passwords.

Still, in order to do any damage to the application/platform the hacker should be aware of procedures for remote work (administration/support/development) in order to log-in and make a mess.