1) India as a whole is overpopulated and has a pollution problem. This particular issue is limited to Delhi, however.
2) From the perspective of the magnitude and extreme levels of pollution, this is a seasonal issue whose primary contributor, by a large margin, is farmers burning off their crop stubble after their harvest ends. This has been happening every year for some time (more on this below).
3) Political ideologues have hijacked this issue to push their pet issues like automobile emissions and firecrackers. These are obviously important issues which most reasonable people will agree ought to be curbed. Sadly a lot of otherwise intelligent folks have a blind spot when it comes to these issues, so there is a temptation to let it slide when these kinds of politics are being played dishonestly, even though their motivations are different. By emphasizing one's biased issues, the root cause is ignored, which helps nobody.
On stubble burning:
There was a time when sowing paddy in the neighbouring state of Punjab began during the first week of June. Harvest season would be mid October, and sowing would resume in late November.
During the period between October and November, farmers would let the stubble decompose over time. This required a lot of water and time.
Unfortunately this took a toll on groundwater levels. Government intervention led to a reduction in water supplies to the fields during the summer (June), when there was a shortage of water across the state. This led to a delay in the sowing season (moved to July). The entire crop cycle got moved up, and so the time previously available to allow decomposition of stubble had reduced to barely a few days. Their solution to this problem was to burn all the stubble.