The use of boner as a metaphorical term for excitement, with a play on words of bro.
A broner is the excitement a man receives when he's informed of a bro related event that the man is eager to experience. When a man gets a broner, he is not erect it is merely a metaphor for excitement.
Bro #1: "I got us tickets to the Superball!"
Bro #2: "Dude, I just popped a broner."