bifurcation, fork
丫 centers on a branching point where something splits into two: a fork or bifurcation. The shape itself suggests a dividing path.
丫 is a pictograph of a fork or branching point, with the two upper strokes representing the split and the vertical stroke the stem.
The two short strokes at the top look like a fork in a road or a tree branch splitting. The vertical line is the main path before it divides.
For ア, imagine a fork in a road marked by a large letter 'A': the 'A' shape echoes the branching top of 丫 and cues the reading ア.