The dynamics of the scanning tunneling microscope (STM)-induced desorption of hydrogen atoms from Si(100)-(2 x 1):H (D) surfaces in the "above electronic threshold limit", which is believed to proceed via a short-lived sigma --> sigma* resonance, is investigated theoretically. We use both time-dependent wave packet and density matrix methods and compare them to classical mechanics. Isotope effects in the desorption yields and desorbate translational energies, the role of coordinate-dependent electronic relaxation, and finally the dependence of the observables on the surface temperature T-s are addressed. In agreement with experiment we find no significant dependence of the isotope effect in the desorption yield up to T-s approximate to 300 K. However, for higher T-s a substantial decrease of the ratio between H and D yield is predicted.