Why do wing tip vortices tend to be most dangerous when there is a light quartering tailwind?
According to the FAA's Pilot and Air Traffic Controller Guide to Wake Turbulence (section 2.4.5) it's because:


Light: in light winds the vortices stay around longer; if the wind is greater than 5 knots then the vortices move out of the way quickly and break up
Quartering: crosswinds blow the vortices across the aircraft's flight path, which means that if you're landing next then one vortex might still be over the runway. With no crosswind at all, the vortices will dissipate on each side of the runway, out of your way.
Tailwind: the vortices are blown in the same direction as the aircraft, which means that the vortices from a landing aircraft will be blown down the runway rather than away from it. Again, that's an issue for the next aircraft landing.


They have a handy diagram to illustrate the vortex paths from an aircraft at 0, 3 and 6 knots. You can see that a light, 3-knot crosswind causes one vortex to still be in the flightpath after 20 seconds, whereas with 0 or 6 knots of wind the vortex is out of the flightpath completely. You can't see tailwind on this diagram, of course, but the principle is the same: it blows the vortex down the runway (flightpath) rather than away from it.