2nd Coldstream Guards - May - June 1916 - researching Private 15047 Alfred Reader KIA 20/06/16
"20 Jun 1916 Battn relieved about 10pm by the 3rd Battn Coldstream Guards and returned to reserve dugouts. Casualties OR Killed 4. Wounded 4 (2 remained at duty) 2nd Lt Butler

? joined from England."
Assuming Pte Reader was one of the 4 killed it was most likely from shelling, perhaps sniping.
The diary does not give trench details but the reserve dugouts were on 'Canal Bank' near Ypres. They went into the trenches on 16 Jun relieving the 14th DLI. May/June was what passed for a quiet period, a couple of stints in the trenches, some working parties but mostly in billets - they even had 'battalion sports' for a couple of days while in billets at Quelmes (26 May -6 Jun).
CWGC has 5 Coldstreamers dying that day - all buried at Essex Farm (an Advanced Dressing Station famous as the place where John McCrae wrote "In Flanders' Fields").
I'm not sure if anything should be read into this burial place - most of the dead would have reached this point alive before dying of their wounds. However, it would have been pretty close to the front and hence a convenient cemetery to bury comrades falling as the battalion withdrew from the trenches.