Not such a rare event from the point of view of the British flying services.
According to the breakdown in Henshaw's work, in the 52 months of war there were 32 months (62%) when British losses east of the lines exceeded 50% of the monthly casualty total.
The months when this occurred were:
September, October, December 1914;
February, March, July, August, September, December 1915;
January, February, March, September, October, November 1916;
April, June, July, August, September, October, November, December 1917;
January, March, May, June, July, August, September, October, November 1918.