A straw boss is generally second in command or perhaps an even lesser supervisor. I haven't found a resource that indicates the reason for the use of the word, "straw" to differentiate this foreman from the primary boss. I suspect, though, it is one of those odd terms, developed particularly for the purpose in much the same way as "grass" is used in the term, "grass widow" to differentiate a divorcée from a woman who lost her husband by way of his death.