My best guess would be that Bruce felt increasingly trapped by the changing direction that the band was trying to negotiate--and probably frustrated by the fact that Carl and Dennis were getting a lot more "air time" in that process than he was. But let's look at what he was contributing to the band song-wise during the 1969-71 period: "The Nearest Faraway Place," "Bluebirds Over The Mountain," "Tears in the Morning," "Dierdre," and "Disney Girls." This is not a discography suggesting that Bruce is in any way central to what was happening with the band at that time--a time of great creativity but also of great ferment.
Bruce was by this time a very experienced and proficient (if not particularly inspired) producer, and all of the tracks he produced that wound up on 20/20, SUNFLOWER and SURF'S UP are rock-solid from a technical perspective, but they are really marginal in terms of their potential contribution to a viable new artistic/commercial direction for the band. And this fact had to be pretty sore-thumb obvious for some time--probably since even before he began submitting material. At that time he was looking to have things both ways, oscillating between loyal subordinate and "objective" outsider. "Ten Years Harmony," which IIRC was the track he was pushing for inclusion on CATP, is the type of track the band would do later on, when it acquiesced to forms of self-referential nostalgia.
Being a "schmaltzmeister" at a time when the band was competing for a slice of credibility in an age when hard rock was at its apex was not going to sit well--and when Dennis' "orchestral schmaltz" was given a place at the table for CATP, and when Blondie and Ricky were tapped for two songs to integrate them into the band, and there was even a chance that another pop-oriented outside writer (Don Goldberg) might even slip past him onto the LP, it's not all that surprising to discover that Bruce acted out with some passive-aggressive behavior. It was probably clear to him for some time that he was on the margins, and he probably compensated by putting on some of the Beverly Hills airs that he and Terry Melcher had due to their more privileged upbringings. That, too, was (and probably still is, deep in the back of his mind...) part of how he held himself outside the group, and has clearly continued to do so even after coming back in '78 when the Wilson brothers were too burned out to take the controls in the studio.
GREAT post.
It's quite amazing when you think about it that Bruce got *four* lead vocals (all or partial) on the SUNFLOWER album when up until that point he hadn't had a proper lead vocal at all on a released track. Particularly surprising that he got handed the lead on "At My Window" (which he did beautifully, perhaps his best lead vocal for the band) which was written by two other members of the band. Clearly, something was going on politically with Bruce in the band around this era and that political situation was then resolved when he left the band a year or so later.
Bruce's career is fascinating. His time outside the Beach Boys was extremely productive. Just as before he joined the band, during the mid '70s he was involved in various ways with quite a number of hits besides the obvious one with Barry Manilow.