As others have indicated, this is a common issue and easily remedied problem. We use a three-step power sequencer on our church system, and I use a similar one with my home theater, just to be safe. You may also have to add some type of remote switching device to power up systems that are not local with the mixer.
Most modern power amps and powered speakers have a relay that connect the transducers (speakers) after the amp has stabilized, but most mixers do not include such circuitry, so you must follow the rule to connect power amps AFTER the mixer and any other inline devices have powered up.