Hi Lewis
Sorry no-one has answered your post as yet, but there is no definitive answer that just pops into my mind.
If I had to guess...I would say they could be right about the injector leaking back with a little higher pressure than normal leak back.
My thoughts are that this is not the sort of pipe that should have enough pressure going through it to push it off the injector, no matter what sort of clip is securing it.
Let us know what you find out and how it goes
Cheers, Trevor