I can't tell you what killed the Strahan brothers during the spring of 1882 --- Thomas A. at the age of 24 on April 26 and William F. on May 18 at the age of 22. No obituary or death record has survived.
The Strahans were prosperous farmers near Oakley in Liberty Township, northwest of Chariton, enumerated in the 1880 census with their mother and a hired hand. They had arrived in Lucas County during the late 1860s.
But I can tell you a little about their tombstone in the Chariton Cemetery, where their mother, Mary, had arranged for their burial next to their late father, Alexander, who died during 1869, courtesy of a brief news item published in The Chariton Patriot of August 2, 1882:
"Moard Bros. of Burlington, manufacturers and importers of foreign and American marble and granite, have just put up an Italian marble monument in the Chariton cemetery for Mrs. Strah(a)n which is a model of beauty in design and workmanship. It is sacred to the memory of William and Thomas Strah(a)n, brothers who died during the summer and whose taking off was within a few weeks of each other. Any one desiring work of this kind will consult their best interests by addressing Messrs. Moard Bros. at Burlington."
The Moard brothers were widely known in southeast Iowa at the time, apparently specializing in the "arched" design evident in the Strahan stone. Here's a report from The Burlington Daily Hawk Eye-Gazette of Dec. 18, 1881, and an image from Find a Grave of the monument to the Dill family mentioned in it:
"They are crowded with work. From every quarter orders are pouring in upon them. The works are running at full blast. They are now working a large family monument, ten feet high, for Mrs. Dill, of Blandinsville, Illinois. This is an arched monument and will be inscribed to Mr. and Mrs. Dill. Another monument, costing $650, was in preparation and its destination is Shelbyville, Missouri. The order was received this week. The chisels, mallets and pencils in these works are cutting, knocking and drawing as lively as muscle can make them. The present week work to the amount of $2,000 has come in quick succession. Specimens of worked marble and of rare artistic beauty are displayed from their show room. This firm wins a strong foothold everywhere its excellent workmanship is introduced. By their work they are judged."
The Moards seem to have been effective self-promoters, both by the quality of their work and skillful use of the media.
Back in Chariton, the heirs of Mary and Silas Cory --- who died during 1882 and 1883 respectively --- seem to have been impressed enough to order something similar for their family graves, just south of the Strahans.
By now the latter stone has lost the urn that probably topped the arch originally, but it's obvious that the two tombstones had the same designers.