My great uncle was a territorial prior to the war and then saw service in Egypt, Gallipoli and France. He was released as "time served" early in 1916. The Military Service Act came into force around this time effectively meaning all men (time served territorials included) were liable for conscription. In a bid to avoid the trenches again he volunteered for the RFC. I wonder if this may be the case with your man?
In my great uncle's case he qualified to become a pilot of F2b's and ultimately was shot down twice, perhaps he would have been safer keeping his down in a trench! Despite this mishap (and arriving home from Gallipoli on a hospital ship and a spell in military hospital with jaundice), he lived until he was 97.