I am in the BS camp that the pump wouldn't just start, fill the line, create a siphon and it would drain as Parks mentioned...otherwise the reverse would happen.
My relatively new shower sump by Rule had a badly designed check valve right at the pump exit of the sump. The pump couldn't push past it. Got rid of it and now the pump cycles a few more times with drains back...but at least it pumps overboard.
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