The Brief: "Shots Fired" is often used an exclamation, following a diss.

DEEP DIVE
When someone blatantly calls out or disses another person, a natural response might be to say, “Shots Fired.” When said IRL, it’s often used as an exclamatory statement like, “Oof, Shots fired!”
It’s sometimes used with hashtag on social media (#shotsfired) and there’s often an implied sarcastic tone.
My own twin doesn’t even follow me @moser_caitlin #shotsfired
— bdoggy (@bdoggy6) March 11, 2019
More often than not, it’s usually the person/group who has been insulted that acknowledges the diss with this statement.
How many 'Nice Guys' does it take to screw in a lightbulb? 💡
None. They just compliment it and get pissed when it doesn't screw.#funny #NotAllMen #shotsfired pic.twitter.com/guuJEhbQVQ
— Dear Catastrophe Leah (@wilde_thing) March 12, 2019