What's causing these drops?

Hey all

I play Overwatch normally on 120 FPS and I still get FPS drops twice a round ; sometimes the FPS drops and then also disconnects me from the round, sometimes it just brings me to 30 for a couple seconds during a big team fight. I don't think lag behaves that way ; lag would be a delayed server response to any action I did in the game, correct?

System ;
AMD FX-6300 (4.7 GHz)
8 GB DDR3-1333
Radeon RX 470