As early as 1996, rumors periodically surfaced that Anthony had been fired from Van Halen, despite claims to the contrary. He continued working with the band. These rumors persisted until his final departure after the 2004 reunion tour with Hagar.  Anthony's involvement in recording the 1998 album Van Halen III was dramatically less than previous albums. Anthony played bass on only three songs, with Eddie Van Halen playing the bass parts for the remainder. Anthony is credited as a songwriter for the album along with the rest of the band, as is always the case for Van Halen albums. Anthony performed with the band for the 1998 tour, and was credited on messages from the band thereafter. He participated in the band's three reunion efforts with David Lee Roth in 2000 and 2001. Anthony's name was also credited in a few band newsletters and appeared in band interviews during this time. Sometime after this, Anthony disappeared from public view until the 2004 reunion.  In interviews, Eddie and Alex Van Halen suggested they were jamming and writing/recording new material during this time period but appeared to be working without Anthony.  Anthony began periodic appearances with Sammy Hagar during his solo tours. He usually played as part of both the Waboritas and Los Tres Gusanos, two of Hagar's bands. During 2002's David Lee Roth/Sammy Hagar tour, both Michael Anthony and ex-Van Halen vocalist Gary Cherone made guest appearances at concerts, sometimes together.  In 2002, Anthony, Hagar, Neal Schon, Deen Castronovo, and Joe Satriani formed the "supergroup" Planet Us and Anthony began making more frequent performances at Sammy Hagar concerts. Planet Us recorded two songs, including "Psycho Vertigo," which was intended for the original Spider-Man soundtrack but ultimately did not make the album. That and the other Planet Us song written for the band, "Peephole," were later released on the 2008 Sammy Hagar solo album Cosmic Universal Fashion.

Answer this question "what caused his role to start diminishing?" by extracting the answer from the text above.
As early as 1996, rumors periodically surfaced that Anthony had been fired from Van Halen, despite claims to the contrary.