This article covers it briefly
PFI is a radical islamist group, formed after another such group was banned(SIMI). They have links to many terrorist organisations, from Al-qaeda to ISIS. They were mad at some old leader was jailed. So, they were planning something big. NIA caught wind of that and they were going to start crackdowns.
Earlier this week, they attacked multiple RSS and BJP members' houses at night with petrol bombs, at the same time. It was a coordinated attack, iirc at 19 different attacks. The police started cracking down throughout the country, they found some really messed up plans they had. It kind of snowballed into a large issue. Like this document. Now, DMK is claiming that BJP threw petrol bombs against itself for sympathy points. Funny thing is Annamalai actually warned Stalin to take action against violence towards BJP and Hindus or the Centre will intervene.
Some of the issues are still cloudy, naa twitter la thaan pakuren. We all knew PFI is fucked up, there have been multiple tiktok-style videos of radical muslims doing their thing in mallu in the past. It's really not a surprise that they were planning all these.