If they have to drain the pressure down then that's usually because the pressure has built up higher from the expansion of the cold water when heated. The Pressure Reducing Valve could need adjusting or the expansion tank may not be working correctly. I do not see how shower usage would effect the backflow preventer except that a fast shut off of a valve downstream can result in spitting from the back pressure.
Backflows are to be inspected yearly and parts replaced every few years. They are a problem with spillage and require drain piping to an approved drain. If the downstream pressure is more than two pounds lower than the incoming pressure then the backflow preverter is designed to relieve that pressure.
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