It's generally accepted that Roth left the band to pursue a solo career, however that may not be the real story.
To get to the point of the Van Halen brothers, Eddie and Alex, feuding big time with Diamond Dave, we need some context.
Now according to a former Van Halen employee, Pete Angelus (former Van Halen lighting director, creative consultant and Black Crowes manager), the band had decided to take a year off and recharge after pretty much consistently touring since the first album came out. During that break, Roth was going to work on a movie, Crazy From the Heat, (it was never made).
Despite all band members being on board for the hiatus, according to Angelus, a story appeared in Rolling Stone (in July 1984) saying Roth had quit the band. Now according to Angelus, Alex Van Halen told him the band didn't want to wait around while Roth made the movie and that it was "bullshit ... we're not going to be in a holding pattern to Dave's whims."
Talk to the Van Halen brothers and they'll yell to the top of the mountain Roth left the band.
In early 1985, there were signs that all was not well in the Van Halen camp.
Roth released his EP Crazy From the Heat in late January 1985, a four-song collection that contained nothing written by Roth. They were all covers. Then in February, he was quoted as saying "We’ll be going back in the studio and start arguing again and we all look forward to that. ... We have a lot of respect for each other and get along quite well, actually.”
Not true. They weren't getting along at all.
According to reports Roth and Eddie met in March to go over the future of the band. One thing they surely chatted about was Roth making the movie. Turns out he wanted Eddie to do the score and soundtrack. Eddie said "no".
Concerning that meeting, Eddie told Rolling Stone in a 1986 article the discussion came to a grinding halt, with Roth declaring, “I can’t work with you guys anymore. I want to do my movie. Maybe when I’m done, we'll get back together.”
So from Eddie's point of view, it appears there was no talk of a year's hiatus.
Now it should be noted the Van Halen brothers fired their manager, Noel Monk, in April 1985, despite desperate pleas from Roth to keep Monk on board. With no intermediary between Roth and the brothers, things quickly fell apart. With no glue keeping the egos in check, there was no hope of Eddie and Roth reconciling.
Van Halen Likely Broke Up in March, 1985
And publicly, there was no talk of the band breaking up, even by April 1, April Fool's Day and the one most have pegged as the day Diamond Dave departed.
The first public knowledge came in the summer of 1985, when Rolling Stone noted on July 4 that "Van Halen is on permanent hold. Eddie, who’s rumored to be scouting around for a new lead singer, is writing songs with Patty Smyth and planning to collaborate with Pete Townsend. As for David Lee Roth, he intends to pursue an acting career full time and is developing his own movie."At that time, neither Eddie or Roth had said anything publicly.
But the silence from the Van Halen camp ended in August when Eddie declared, in Rolling Stone: "The band as you know it is over. Dave left to be a movie star . . . He even had the balls to ask if I’d write the score for him."
The guitar virtuoso went on to state: "I’m looking for a new lead singer ... it’s weird that it’s over. Twelve years of my life putting up with his bullshit."
So clearly there is a discord between the brothers and Roth and whether or not they were taking a year off, as Angelus stated, or were breaking up because Roth wanted to be a movie star (as the Van Halen brothers have said).
The inclination is to believe Eddie and Alex. Roth is a master of spin control and verbal diarrhea. And it's interesting to note Angelus went on to become one half of the Roth solo video team as a one of the Fabulous Picasso Brothers, so clearly his best (monetary) interests were tied to the Roth camp.
The Van Halen feud became one of the most notorious in rock and roll.