The Rocker

In the late 1980s, Robert "Fish" Fishman (Rainn Wilson) is the drummer in Vesuvius, a fictional heavy metal/glam metal band from Cleveland, Ohio on the verge of making it big. The band is offered a recording contract with one condition - Fish must be replaced as drummer with the president of the record company's nephew. After initially refusing, the band caves when their manager tells them they would get to tour as the opening act for Whitesnake.
20 years later, Vesuvius remains an immensely successful band, while Fish is working a mundane office job, which he is fired from for attacking a co-worker who tried to force him to listen to Vesuvius' new album. After being kicked out by his girlfriend, he is taken in by his sister Lisa (Jane Lynch) and her family. Matt (Josh Gad), Fish's high school-aged nephew, plays keyboards in an alternative rock band called A.D.D., along with his friends Curtis (Teddy Geiger) and Amelia (Emma Stone). The band is scheduled to play their school's prom but the gig is in jeopardy when their drummer is suspended from school. Matt convinces the others to allow Fish to fill in, but he ruins the gig when he launches into an impromptu drum solo. However, Fish is so excited by Curtis' songs and the chance to play again, he convinces them to let him join the band if he can deliver another gig. After repeated failed attempts, he finally succeeds in securing a gig at a club in Fort Wayne, Indiana. Because the other members are all minors, they have to sneak out to the gig, taking Lisa's car without permission, resulting in the band being arrested.
When Fish is kicked out of his sister's house and rents the sweltering basement of his favorite Chinese restaurant, the band invents a new way to practice via four-way iChat. To the disgust of his bandmates, Fish performs in the nude to try and stay cool. The video of the practice is leaked and quickly goes viral under the title of the "naked drummer band". The band is signed to a recording contract by the same label as Vesuvius, and are sent on a midwest tour. However, Fish commits stereotypical acts, despite the physical costs on his body, and he vandalizes a hotel room, which again results in the band's arrest.
After bailing them out, Kim, Curtis's mother (Christina Applegate), promises the other parents she will stay for the remainder of the tour, so their children will not be influenced by Fish's antics.
The label asks A.D.D. to open a show for Vesuvius honoring their induction into the Rock and Roll Hall of Fame. Fish refuses to play the gig and quits. Curtis eventually convinces him to put aside his grudge against his former bandmates and play the show. Fish meets Vesuvius before the show to discover they have become vain and arrogant, using faux British accents, and actually do not remember Fish until one bandmember says he faintly remembers. Discovering that he is able to move on, Fish wishes Vesuvius a great show. Fish and the band perform to a standing ovation. After their gig, Amelia and Curtis, and Fish and Kim both begin a relationship. Vesuvius takes the stage, and during their set the lead singer's microphone falls off the stand while the voice track of their song continues, then one line of the song starts incessantly sounding like a broken record, revealing that they have been lip-syncing. The audience condemns Vesuvius and chants an encore for A.D.D. Over David's (the band's manager) objections, the band dismisses him and performs for the crowd once again.

Question: Why does Fish loose his job?