I tend to agree with AM, either they are new drivers, have inferiority complex or are ever in a rush competing with themselves. Drive to Thika road on a Saturday afternoon and you'll count about 10 crazy Probox drivers.
I once drove behind one on my way to Magadi one evening, he wouldn't give way so I made him push his car to the limit. Poor guy I ended up helping out the guy with water after it heated up.
My two cents, it doesn't matter which car you are driving but if a guy drives up to your rear bumper, give him / her way it means they were driving faster than you were.