From the department of "duh": Ever wondered why rock 'n roll concerts rarely start on time. Via Radley, here's someone who's uncovered the truth.

But, Witts explains, then "there is a curious, ill-defined period between the
moment the technicians have finished their on-stage preparations and the moment
of the band's arrival."
He contends that "it seems to be implicitly accepted
by management and the audience that there may be a pause of up to 20 minutes
following the end of the roadie cabaret."

Seriously, the causes for delay are pretty predicatable, including band members filling various needs, be they chemical, sexual or pshychological. It's not like they'e all superheros who may have to battle aliens or other arch-villians while the opening act warms up the crowd.

At least they're not all lost backstage.