Unfortunately, his detailed service record has not survived.
Another source(as Nanny Jan points out) to look at is the war diary of his battery. The Effects Register and his medal record show it as 152 (not 125) Heavy Battery Royal Garrison Artillery (the Hackney Gunners) who went to France in April 1916.
Thanks to this web site
http://www.hackneygunners.co.uk the war diary can be read here
http://www.hackneygunners.co.uk/wp-content/uploads/War-Diary-Full-Landscape-Watermarked.pdfThere is nothing recorded in July relating to an accident. Had it been related to a gun or to ammunition, I am sure it would have been recorded as that would affect the battery capability so (guesswork follows) an accident involving horses, vehicles, heavy stores are the sort of possibilities.
Generally, it was important, where it could be established, to ensure an accidental death was recorded as such. The core of it was that it was not by enemy action. I do not know, but there may have been an effect on any pension claim. I have seen mention of a death cert recording killed in action when the Effects Register says accident so wouldn't like to guess what the cert might say.
MaxD