Almost Summer Director, Marty Davidson, contacted the BB's to do the soundtrack - but the soundtrack rights/distribution what have you was with MCA, and the BB's exclusive CBS! contract didn't allow it. However, individual members could be involved thus Celebration came into being. According to per Ian and Jon's book, p234/235 (far left and far right hand side text respectively).
And of all the times that this unusual event happened, it was a bizarre stroke of odd luck that this one time forced to release a track under a pseudonym, that Brian writes one of his most catchy throwback type of songs (which would probably have been a hit if released as a "BB" track).
If I didn't know any better, I'd almost think this was a passive aggressive move by Brian to deprive "The BBs" name from having his better material. Not sure if that's actually true, but it's a very odd coincidence that this all happened this way, in true oddball BB fashion.