When does a question appear subjective and is likely to be closed?

I asked a question recently, and when I was done typing the title, I got a pop-up saying:

The question you're asking appears subjective and is likely to be closed.

Out of curiosity, what are the criteria for getting this pop-up?

My question was titled "Sharing a pepperoni pizza with your worst enemy", and my guess is that "worst" was the trigger here. That or "pizza".

• If pizza is persecuted then we are all doomed. – rschwieb Jan 29 '16 at 10:01
• And now I want pizza... – Najib Idrissi Jan 29 '16 at 10:09
• That automated warning may not exactly fit certain mathematical problems. Its inner workings do not recognize that your title refers to a mathematical problem. So go ahead. Ignore the warning. – GEdgar Jan 29 '16 at 16:35