Kessler syndrome presumably?

Keeping the orbits low enough, and/or intentionally going suborbital after docking/before starting the fuel transfer, will make the chances of that being possible very low.

It's also worth considering that they have demonstrated cryo propellant pumping between two tanks within a ship, so, AFAIK, transfer between two ships is more about testing the docking systems, than it is about the pumps. They could probably rig the system to first pump some inert gas to verify the quality of the docking, then try to pump propellants.

...caused by what?