By default, missiles are programmed to target the Indian ocean in peacetime. So even if you accidentally launch it it will not go to enemy territory.
Missiles don't necessarily travel in a straight line. You can program them to go to point 1, point 2, and then to the target.
To launch a Brahmos missile to another pre planned point, you have to enter the new coordinates. And during tests, the coordinates are ALWAYS verified, especially when testing near the border.
Some nuclear missiles have preset coordinates for major enemy cities. It's possible some idiot set it to a preset target in Mian Channu, but then it would have hit some target of value. Unless of course it was meant for a nuclear warhead, in which case these coordinates would be chosen for maximum effect of an air burst.
A faulty guidance system would see the missile spiralling out of control, not going on a steady supersonic flight at 40,000 feet and making a very precise turn.
Brahmos has a self destruct procedure in case all else fails.
Judging from this and the path of the Brahmos missile, it's extremely difficult to brush this off as some accidental technical glitch.
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Also, the fact that frantic hotline calls were made to Pakistan is likely the reason no swift response was undertaken.