At 120 MHz, the wavelength is 2.5 meters. The 15 meter cable is six wavelengths long. I would expect the longer cable to radiate more. However, you coiled it which acts like a big inductor or choke to keep that from radiating. I would expect if you uncoiled that loop, it would increase in emissions.
I have also seen test cables built a lot better than production cables. It could also be how they grounded the shields at the connectors. A pigtail ground would really increase the radiation.

I guess your shorter cable, in combination with height above ground plane and LISN termination is a more or less perfect radiator for 120 MHz. How is the shielding of the cable terminated at the DUT's end and at the LISN ? Is the shield open at the LISN, the shield itself may become a resonator.
And, as Tom Fagan wrote, a pigtail grounding of the shield may act as a good injection point, especially if connected to signal GND on a PCB inside DUT.
The shielding shall be a extension of the faradays cage that encloses the DUT, connected to the shell of the shielded connector.

I find it often a lot easier to filter the RF away within the DUT than try to prevent the cable from radiating.
120 MHz is not that hard to kill with a CM + DM filter. Got a current probe ? Make a breakout cable and measure the RF current in the cables at the DUTs end, measure DM and CM and try to figure out what filter type you need.