"I will walk about on the
heaving grass rather shakily"

The speaker of the poem has a hangover; the line break enacts a "lurch," mimetic of his unsteady gait. Later on the movement of the lines mimics the clumsy flight of the model airplane, or the act of attention of the speaker watching the irregular flight of the plane. Such effects can be overdone, of course (here they are not, in my judgment), but they are clearly not random or arbitrary.