Interesting, a similar thing happened to me yesterday. A car was right biased in the right lane (red light) with turn signal on and turn actuall slightly started, driver looking to left for gap in traffic to turn.
I pulled up after them to their left. Light turn green and surprise as I went forward the car turned a bit back to the left and went straight. I let up on my accelerator and dropped in behind them.
However a very big difference in this situation is that the driver was sheepishly appologetic, the waved and smiled to me, clearly a non-verbal 'i'm sorry' communication. Anyway, I think its OK for drivers to change their mind as long as its not dangerous.
What the OP dealt with may have been similar except the driver responded agressively. It happens, nothing you can do except be alert and prepared for the unexpected.

Al