Overtaking in Indian traffic

I've noticed one really annoying and scary thing in Indian traffic. It's got to do with the way drivers of 2-wheelers overtake other larger (usually) vehicles. They overtake the car or truck/lorry/bus and then immediately cut in front of it, completely ignoring the fact that the other vehicle is also moving at a fair rate of knots. This forces the "overtakee" to slow down abruptly to avoid hitting the 2-wheeler that just overtook him/her. I'm curious to know whether this is caused by a) complacence of the driver of the 2-wheeler b) complete ignorance of basic traffic rules/etiquette or c) complete callousness of the possible consequences.