Final answer:
Murry Wilson, the father and then-manager of the Beach Boys, renamed the band to 'The Beach Bums' without their consent in 1969 for their '20/20' album, which the band opposed and eventually reversed.
Step-by-step explanation:
The person who re-named the Beach Boys behind their back and without their permission was Murry Wilson, the father of band members Brian, Dennis, and Carl Wilson. Murry Wilson was also the band's manager at the time.
This incident occurred in 1969 when Murry, without the band's consent, submitted a revised version of their album '20/20' to Capitol Records under a new band name, 'The Beach Bums'. The band was not pleased with this unauthorized change, and the name was subsequently reverted back to the original Beach Boys.