1. Why are FIIs selling Indian stocks so vigorously?

Well, a number of factors. It's actually a domino effect.

And it all begins with the same story…
The day FED decided to go hard on inflation.

2. In layman terms, FED regulates the flow of dollars in the world. For last many years, after the 2008 crisis, they have been printing dollars recklessly to help US Govt fund several economy stabilizing projects by buying US bonds.

3. US 10Y, 30Y bonds are basically a kind of internal debt that the govt raises to run its projects as it has been running into huge deficit for quite sometime.

4. That, along with near zero interest rates incentivizes banks to lend more to whoever is credit worthy. These credit worthy people use the opportunity to raise dollar denominated debt and convert them into local currencies and invest in emerging markets (among other things).

5. Most EM currencies continue to depreciate against USD as most of them, including India, are net importer of goods and services. That is, they spend more USD than what they earn. So when a foreign investor invests in an EM, it has to factor in this depreciation as well.