  Recent observations with JWST and ALMA have revealed extremely massive
quiescent galaxies at redshifts of z=3 and higher, indicating both rapid onset
and quenching of star formation. Using the cosmological simulation suite
Magneticum Pathfinder we reproduce the observed number densities and stellar
masses, with 36 quenched galaxies of stellar mass larger than 3e10Msun at
z=3.42. We find that these galaxies are quenched through a rapid burst of
star-formation and subsequent AGN feedback caused by a particularly isotropic
collapse of surrounding gas, occurring on timescales of around 200Myr or
shorter. The resulting quenched galaxies host stellar components which are
kinematically fast rotating and alpha-enhanced, while exhibiting a steeper
metallicity and flatter age gradient compared to galaxies of similar stellar
mass. The gas of the galaxies has been metal enriched and ejected. We find that
quenched galaxies do not inhabit the densest nodes, but rather sit in local
underdensities. We analyze observable metrics to predict future quenching at
high redshifts, finding that on shorter timescales <500Myr the ratio M_bh/M_*
is the best predictor, followed by the burstiness of the preceding
star-formation, t50-t90 (time to go from 50% to 90% stellar mass). On longer
timescales, >1Gyr, the environment becomes the strongest predictor, followed by
t50-t90, indicating that at high redshifts the consumption of old and lack of
new gas are more relevant for long-term prevention of star-formation than the
presence of a massive AGN. We predict that relics of such high-z quenched
galaxies should best be characterized by a strong alpha enhancement.
