"¿Quién decidió esto?"
Translation:Who decided this?
Actually it's not an uncommon thing to say in a Bureaucratic situation. While the auxiliary verb do is a fundamental part of any question without an interrogative pronoun and most negative statements, do is also introduced as an emphatic element. In a question it would be sort of an impatient element. Scenario::
You go into work and every one is gearing up to start a big project. You are surprised because, although the project had been discussed for weeks, your manager had told you that it wasn't going to work so he was not going to approve it. You say this to a colleague who tells you the manager is still against it. So you ask him Who did decide this? In other words it is shorthand for if the manager did not decide this, who did decide it.
But whether this added emphasis wouldn't also be reflected in the Spanish is a separate question. Spanish does use both an emphatic sí or ya in some sentences. I am not sure if either of those would have the same effect.