Latest report is that the pilot accidentally hit the wrong button. That's gonna be an expensive mistake.
This is actually a not-uncommon occurrence. The transponder code for a hijacking is 7500. For lost communication it is 7600 and for a regular emergency it is 7700. It is very easy to confuse these or enter 7500 by mistake. It happens on occasion. The only thing that's new is people freaking out about it.