CA – The driver who rear ended me filed a claim against me.

I was driving down a busy road and the car infront of me was slowing down quickly so I braked, after the car infront of me started moving the car behind me hit me before I could even start driving again.

When the guy got out he just kept asking me why I braked. I just told him we will let the insurance handle it.

I tried to file a claim with his insurance on their website (it’s not a big name company, it took me forever to even find any info on his insurance company and how to contact them).

I found out he filed a claim against me through my insurance. I’m concerned he is trying to lie about what happened. Should I be concerned?

Edit: I am also wondering if anyone else has had a similar situation?