BVM recommends using 6mm OD (4 MM ID) polyurethane tubing for the pump feed line. (It is supplied with the UAT...why not use it?).
It is possible that the nut could be cutting through the wall of the Tygon. Tygon is much more fragile than the supplied tubing.
If you replace the Tygon on that run, you should not have any leakage problems.