The following is copied from Scituate Vital Records, Scituate Genealogy websiste:
"Josiah had a considerable tract of land contiguous to Herring brook hill on the north. His house was in the valley one fourth mile north of the South Meeting-house, opposite to the present  house of Mr. James N. Sparrell. He unfortunately lost his life, A. D. 1693. He was in the act of drying the Town's stock of powder on the roof of his house, when a spark from the chimney falling, his life was instantly lost, and his house laid in ruins."
On 6 Oct 1684 when Josiah was 26, he married Isabel [the widow Witherell], in Scituate, MA. Isabel died on 31 Mar 1689 in Scituate, MA.
Deane mentions in his appendix V. on pg 405 that he 'discovered an important error" concerning the gunpowder incident , and recounts the story of Lt. James and the gunpowder in 1665.
He doesn't correct the date of death for Josiah, however.
Pratt points out the errors in his work on pg 322 but offers no other details on Joshiah's death, other than he "was not blown up by the town's gunpowder". pg 324