[en] Molecular-dynamics simulations have been performed for frozen Rydberg gases in a strong magnetic field of B = 2.9 T, up to a microsecond time scale. We found that a small number of spontaneously ionized electrons may de-excite a large portion of Rydberg atoms, while the free electrons leave the frozen Rydberg gases along the B-field. Such de-excitations as well as Rydberg-Rydberg atom interactions can induce significant heating to strongly magnetized Rydberg gases. Kinetic energy increase by 2-3 orders of magnitude has been observed for the initially frozen (∼0.2 mK) Rydberg gases. (fast track communication)