Vimy Ridge was actually the front line; and if he had disappeared behind it,
that doesn't mean he was that far behind the lines.
So it would have taken some time to climb to a descent altitude.
(I didn't get, what aircraft he was flying; I suppose, an Albatros D III)
It could be safer to remain low, than to climb to an altitude, that was still
no advantage against enemy fighters, but made you more visible to high flyers.
And I think, as long as they were never hit by ground fire bullets, they felt very
superiour, very safe up there. An Albatros is a solid aircraft; you are surrounded
by a wooden fuselage, and it may give a pilot the idea, he could not get seriously
hurt by a bullet. Only after that hit, he would have never felt the same about it.
But this is just my feeling about it.