The verb σκανδαλίζω (translated 'fall away') is a matter of stumbling over an obstacle, thus the concept is of being tripped up oneself, by circumstances, or of tripping up others by one's behaviour - in deeds and words.
One scandalises, that is to say one causes a 'scandal' causing others to be stumbled in their spiritual progress by one's own departure from truth and proper behaviour. It sets a bad example to others and confuses those who might, before, have been following in one's footsteps or just accompanying one on the path of truth and holiness.
The 'stumbling' and 'falling' is the opposite of uprightness. Instead of an upright walk, one has tripped and fallen. One is no longer upright. And it is a public scandal.
The roots did not go down deep enough to give stability. Because of the stoniness of the heart, there was no depth into which roots could penetrate the heart. Thus it was all on the surface. The word did not go down into the depths of the human heart, to give strength, to stabilise.
And at the first sign of upset, of turbulent circumstance, of disappointment, of unexpected difficulty or affliction, the candidate has tripped, stumbled, fallen.
And is scandalously no longer upright.
But the figure is not of a walk, a pathway, for the immediate figure is of a plant growing in a field, corn or wheat or similar. In this case, if there is stoniness, and if the roots do not penetrate the earth deeply enough, the plant wilts, the stem cracks and the plant falls over, its top parts now down on the ground, no longer drinking in the sunshine.
The plant is 'fallen away'. Collapsed. No longer upright.
And it is a 'scandal' to see the plant, among its upright fellows, now flattened on the earth, useless.
The lesson is that the word of God should go deeply into the heart and should not be hindered by obstructiveness, by hard-hearted resistance to the word of God. But rather the word should be permitted to sink deeply into the soul, accomplishing the purposes of the Spirit by its penetration.
Deeply, should it go into the deepest recesses of the heart. Rooting downwards into the depths.
Then there will be stability in the arena of the topsoil, strength to withstand the winds that blow and the rains that beat upon the house. As in another figure, that of the house that was built on rock not sand. Built upon hearing and doing (not just hearing, superficially).
These sayings of Jesus are for us to be instructed that we might not be shallow and superficial. That we might endure to the end, and not fall away.