  We investigate the origin of close-in planets and related phenomena orbiting
white dwarfs (WDs), which are thought to originate from orbits more distant
from the star. We use the planetary architectures of the 75 multiple-planet
systems (four, five and six planets) detected orbiting main-sequence stars to
build 750 dynamically analogous templates that we evolve to the WD phase. Our
exploration of parameter space, although not exhaustive, is guided and
restricted by observations and we find that the higher the multiplicity of the
planetary system, the more likely it is to have a dynamical instability (losing
planets, orbit crossing and scattering), that eventually will send a planet (or
small object) through a close periastron passage. Indeed, the fraction of
unstable four- to six-planet simulations is comparable to the 25-50$\%$
fraction of WDs having atmospheric pollution. Additionally, the onset of
instability in the four- to six-planet configurations peaks in the first Gyr of
the WD cooling time, decreasing thereafter. Planetary multiplicity is a natural
condition to explain the presence of close-in planets to WDs, without having to
invoke the specific architectures of the system or their migration through the
von Zeipel-Lidov-Kozai (ZLK) effects from binary companions or their survival
through the common envelope phase.
