Rob Caggiano says he never understood why Anthrax refused to use his skills as a songwriter.
The guitarist quit the band in 2013 and says he became frustrated at being overlooked when it came to writing new material.
He tells Ultimate Guitar: "I never understood it, to be honest. I do understand it but at the same time I was with those guys for a really long time. You would think at some point along that journey, we would have collaborated on something.
"At that point when I decided to leave those guys, I just felt like I needed a change. The thing is the band was doing amazing. Anthrax was on top of their game. We had just come off the Big Four tour and all this awesome stuff happened. But the bottom line was I just wasn't happy and my heart wasn't in it."
Although it took its toll, Caggiano insists he understood that Anthrax had a system and says he doesn't blame them for not wanting to change it.
He says: "The main thing was it was never a creative outlet for me. That's something I'd been thinking about for a while — for years. It didn't happen overnight. It's just something I needed to do for my own sanity.
"In Anthrax my role was, 'You're playing the solo.' Don't get me wrong, I love playing solos. It's what I do. I'm a lead guitar player. But there's definitely so much more I can do and I had way more to say musically than what I was doing with Anthrax. That's basically it."
Caggiano later joined Volbeat as a full-time member, but he remains friends with his former employers.
He adds: "I love the Anthrax guys. They're like my second family. I think they get it and they totally understand why I needed to do that. We're still cool and we still hang out and every time I see them, it's totally cool."