Earth rotation question

I am writing an "end-of-the-world" type novel which the earth's rotation
will change, causing great upheaval on the earth and making the days
shorter. My question is what could cause such a catastrophe?

Could this be caused by:
1. a super-giant solar flare
2. a close fly-by by a gigantic mass (one gigantic mass or thousands of
smaller objects)
3. major impact on the earth by a comet or asteroid (I know, this one has
been done to death)

Any other suggestions?

All responses will be greatly appreciated.

Thank you.
Mike