Auto (commercial vehicles) sales graph - interesting pattern

This chart below shows sales of HML commercial vehicles (Heavy, medium and light commercial vehicles). The data was collected by me; and the chart and pointers given below were given to me by my friend Ankit.

1. From 2001 onwards, a secular rise at a stable growth rate can be seen
2. Every quarter end, the sales rise. Then next month they drop, pick up somewhat in second month and peak to a new high at the end of next quarter
3. As the volumes have increased, absolute swings have also increased. (2001 vs. 2007-08). However, the percentage swing appears to be the same
   

Some explanation of point 2 from an analyst Richard (he covers auto sector) in my company - " Spikes in September and March are easy to explain … they are due to depreciation benefits which a buyer can claim"

My understanding of quarter end spikes (which is actually very obvious, nothing special) - Probably auto manufacturers give some sops / discounts to increase year end and quarter end sales.