Why Hams End A QSO With Dit Dit
In ye olden days, every time a train left the station, it would sound two short blasts from its whistle, warning everyone it was moving out. The old telegraph operators having been exposed to this practice, picked it up and would send two dits at the end of each telegraph message. This meant “GOODBYE”. So ham radio CW operators adopted the practice. It is still used worldwide today.
Source: AC6V’s website