Probably lots of reasons, @TeamCuthbert. Lots of countries have had second/third waves, after all. Part of the problem is that India didn't prepare adequately for this.
There have been far too many mass gatherings - election rallies, religious festivals, huge weddings etc. All of these would have encouraged the virus to spread.
Then there are the new variants. The Kent variant is more transmissible, we know, and that seems to have had a significant impact. Then there are at least a couple of new Indian variants which may be similar.
And it's possible that, due to poor public health surveillance and limited access to health care for a significant proportion of the population, case rates/death rates may never have been as low as they seemed anyway. The virus might have been silently spreading for ages, and it's only now that the effects of that are being observed.