It was intentionally put into the chip. The researches linked the backdoor to the NSA.
"The exploited feature, the cybersecurity firm says, was likely intended for debugging purposes or might have been included by error, as it is undocumented and unknown."
They worded it like that as they dont have evidence. Magic javascript code that disables paging for debugging purposes is like removing the brains of a patient just to visit a dentist.
