Sorry for the late reply. I'm afraid I cant answer your question with certainty but hope this gives some idea about what you found!

Seattle during those years was quite a rough, booming metropolis. It is hard to understate how massively the physical geography of those years was changed, because the legacy is still visible in the city today. Accompanying a massive population move, massive swaths of the city were regraded, ship canals were cut, and lakes and rivers altered. The population of the city roughly doubled during that window of time, and with that came an enormous amount of construction.

Labor safety standards were undoubtedly not what they are today. Washington State had a Bureau of Labor to oversee some basic laws (plumbers, for instance, were required to be licensed at the time) and since 1911 one of the US’ earliest worker’s compensation systems (a result of long years of pressure by unions in the state). But death from work was nevertheless FAR more common than it is today: this 1999 CDC report states that in 1913 23,000 workplace deaths were recorded from a workforce of 38 million while by the late 90s there were a little over 5,000 annually from a workforce of around 130 million. If I’m doing my math right that means you were 15 times as likely to die on the job, and that may be an under-count due as early 20th century workplace death statistics could at times be unreliable. Factor in higher overall mortality rates from any number of factors like disease and violent injury, or the long-term results of working closely with lead pipes, and simply more plumbers would have died at that time.

In the Washington Bureau of Labor’s 1907-08 report, the Seattle Local of the Plumbers and Gasfitters Union (Local 32 of the United Association) reported a membership of 200. If even two of those guys died every year (not impossible given mortality rates at the time) that would fill up the section of the cemetery. But why might they all have been together?

Keep in mind that at the time (and to a lesser extent today) Unions served as mutual aid organizations for their membership. This was a vital function for their members, and was often the only way for them to access things like life or health insurance, or homes for retired workers, setting aside social benefits. If you were too sick to work, your union often covered you with some amount per week ($5.00 in the case of the plumbers according to that 1907-08 report, which was an average wage for “juniors” just starting out, while journeymen averaged $6.50). Many unions, or union affiliated organizations like the German immigrant Arbeiter Bildungs und Unterstützungs Verein, even had a doctor on a sort of retainer for their members.

Plumbers Local 32 also paid out a funeral benefit of $100. Now this is a bit of speculation regarding this particular local*, but it was not an uncommon practice at the time for union locals at that time to purchase sections of the cemetery for their members. In some places one can even find whole cemeteries owned by a union; I don’t have access to my books at the moment, but see, for instance, this article on page 9 in the Carpenters Union magazine regarding a historic Carpenter’s Cemetery in Florida run started in the 1920s. The well known agitator Mother Jones is famously buried in the Union Miners Cemetary, established by the Mt. Olive United Mine Workers local. It is easy to imagine Local 32 paying for a group of plots at your local cemetery as a way to help stretch those $100 funeral benefits further.

*I am quite familiar with a lot of the records of unions held at the University of Washington’s Labor Archives for this period, but unfortunately the only Plumbers Union records available are for the statewide body, and then only from the 1940s onwards, so there’s not an easy way to check whether this was a part of their funeral benefits. A purchase of such a plot might have been mentioned in a short notice in the Seattle Union Record, the local labor paper held on Microfilm at the same library if you really want to go down a rabbit hole!