In my early days I remember having a splice come undone and it was because it was not under tension, and subject to gentle jerks - just like a mooring. It kind of "teased" apart. I now make sure I melt the tails so there is a lump preventing the tail slipping through the strand, and whip securely also. I have never had a splice come undone under tension, so I think the gentle jerking may be the issue here.
I do think the jerking is one of the main problems. However, on Ullswater I would not describe the jerking as gentle as the Ullswater steamers go past!
