Here's a breakdown of the potential reasoning:
* Movement: Ponytails, and the hair ties holding them, "bob" or move up and down.
* Decoration: Some hair ties have decorative elements, also called "bobbles," attached to them. These could be beads, small balls of fabric, or other ornaments that add visual interest and "bob" with the hair's movement.
Over time, the association with the "bobbing" movement and decorative "bobbles" likely led to the widespread use of the term "bobble" for hair ties, especially those that are elasticated.