Why this musician walked off stage mid-song to challenge a man's "disturbing" behaviour.

In the 20 years Brendan Ekstrom, a guitarist for American rock band Circa Survive, has been performing, he has never walked off stage mid-song.

But that all changed earlier this week, when the 39-year-old saw something in the audience of a St. Louis show that was so disturbing that he felt forced to leave.

The guitarist posted a series of messages to his fans on Twitter to explain why he “mysteriously disappeared” halfway through a song.

"Quick note to those who were at the show last night," he wrote to his followers. "I've been doing this for 20 years and I've never walked off stage during a song before.

"For almost a whole song I watched a guy stand a row behind a girl flirting and then trying to kiss her."

Ekstrom explained that as he watched the pair, he became confused as to what was happening.

"At times she seemed OK with his advances but after the second time of watching her push his face and hands away and try to turn...back to watch the show, I asked the security guard to go check on them."

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When the security guard was unclear about what he was asking, the musician felt he had no choice but to confront the man himself.

"I left the stage and walked straight for them, my mind an emotional blur," he wrote. "The man seemed dumbfounded when I showed up...as if nothing was wrong."

After security stepped in, Ekstrom said he wasn't entirely sure what happened to them or what was really going on.

"Maybe they were removed or just moved. Maybe they were in a relationship. Maybe they weren't," he wrote.

"Regardless, what I saw from stage was disturbing to me and completely removed me from the mindset I need to be on stage. It wasn't cute. It was alarming. I'm almost 40 and flirting doesn't look like that."

He acknowledged that having a young daughter added to his anger about the situation.

A post shared by Brendan Ekstrom (@brendanxtrom) on Jun 5, 2017 at 4:35pm PDT

"It made me furious to watch a girl forcibly remove someone's hands from her body multiple times," he said. "I still don't feel great about how I handled this situation. I borderline blacked out in some weird rage mode.

"It makes me nervous to think...about how I would handle situations like this in the future...especially if it is...my daughter."

Fans of the band—particularly women—were quick to thank the musician for taking a stand against what he saw.

"The shoe" oil painting by @theautomaker

A post shared by Brendan Ekstrom (@brendanxtrom) on Jan 29, 2017 at 5:39pm PST

"I appreciate the hell out of what you did," one woman wrote. "Maybe you reacted without thinking but a lot of people around this planet wouldn't even react."

"I'd rather have a show interrupted than watch a fellow human get assaulted," another wrote.

Others shared their own stories of being harassed in the audience while watching other bands perform, adding they wished someone had come to their aid.

"It's so upsetting that men at shows think this behaviour is okay because of how easy it is to get away with it," anther fan wrote.

"I really am so proud that you saw something happening and acted on it."