Why do people's iPhones keep restarting?

Okay so for the past couple weeks my iPhone has just been like restarting when I'm on it. Well it just thought it was mine and then tonight I was on tumblr and there was this post about it and it was like, "reblog if your iPhone has been restating recently" and there was over 55,000 reblogs so that's a lot of iPhones restarting. Why is this happening to everyone?

Someone may have sent you the Effective Power text which does cause that, though there's a fix for it coming in iOS 8.4 later this month.