[quote] i now have to 'defleet' the errant ship, give it orders or stop all orders and then make it rejoin the fleet. [/quote] And did that actually make it stop doing what it did? In my case it made no difference, the ship still tried proudly to jump away.
Gehrschrein
This also happened to me, but I am yet unsure as to what causes this behaviour. At first I thought it was exactly because of what Riptorch mentioned, but then I noticed that the "stray" vessel had its waypoint set not only one jump back, but actually more jumps, going back to the gravity well in which it was produced. I think it is somehow related to the fact that the ship, when it came out of production, didn't actually make it to the shipyard's rally point before being ordered together with it