It was not the work of chance.

More than once he had tested the strength of that old halter rope, covertly, with none to watch, and had felt it stretch and give a little under the strain of his weight; but he had long since learned the futility of breaking ropes so long as there were stable walls or lofty corral fences to contain him.

A moment of local freedom meant nothing, and he had waited until he should find open sky and clear country; this was his reward of patience.
The short, frayed end of the rope dangled beneath his chin; his neck stung where the rope had galled him; but these were minor ills and freedom was a panacea.

Later he would work off the halter as he alone knew how.

The wind, swinging sharply to the north and the west, brought the fragrance of the forests on the slopes of the Eagles, and Alcatraz started on towards them.