Foo Fighters believe there must have been another reason for Justin Bieber cancelling the rest of his 'Purpose' tour.

The 'Run' rockers are considered one of the hardest working bands in the music business and even carried on with their shows when frontman Dave Grohl fell off stage and broke his leg in several places in Sweden, in 2015.

The 'Sorry' hitmaker cited exhaustion for pulling out of the remaining 14 dates of the tour in July, but Foo Fighters' guitarist Chris Shiflett says if it was them they would have completed the gigs because they never pull out of shows unless there is a "really good reason".

The 46-year-old musician told the Daily Star newspaper: "Whatever people say to the press, that is usually something that a publicist has cooked up, and there will be another reason - something behind the scenes.

"I don't want to be judgmental, but Dave broke his leg and absolutely refused to cancel.

"I hate cancelling shows even when there is a really good reason.

"Sometimes s**t happens, sometimes there's a family emergency, sometimes someone gets hurt and it still feels sh***y, so you try not to.

"Luckily I'm in a band where everyone feels that."

Justin's manager Scooter Braun has said that the 23-year-old hunk will reveal the actual reason for axing the shows when the time is right, but insisted he's doing well and is not in a "dark place".

He told 'CBS This Morning': "Again people think he's having issues. What's going on with him? Seems to be falling in another ... dark place. True? Not true?"

"Look, I think he'll decide to tell people when he's ready of what exactly happened.

"But I think he was making a decision, like he said, to protect himself."

Shortly after making the announcement, Justin told his fans - known as Beliebers - that he's taking some time away so he can work on making his "mind, heart, and soul" "sustainable".